Leica II and the FormerSovietUnion clones it spawned
The Leica II [Model D] was the camera that revolutionized system 35mm photography. Introduced in 1932, this was the first Leitz body equipped with a built-in [but separate] 50mm viewfinder [VF] and a 38mm baselength rangefinder [RF] with standardized 28.8mm lens flange to film focal plane registration and interchangeable lens mount capability for lenses of different focal lengths. Lenses offered include the Elmar 35/3.5, 50/3.5, 90/4, 105/6.3, 135/4, Hektor 50/2.5, 73/1.9 and eventually the Summar 50/2. The RF window view is life size [1:1] and easy to focus with shutter speeds from Z [B], 1/20, 1/30, 1/40, 1/60, 1/100, 1/200 and 1/500 provided.

This particular camera started life as a I [A] and an early conversion to II [D] specifications with "lavatory seat" type RF cover , 18mm diameter shutter speed dial and nickel fittings. The lens is a Summar 50mm, f2 also in nickel.
By 1933 it took a back seat to the III [Model F] which was essentially the same camera with the addition of slow speed capability [1/20-1 second] and an even easier and more accurate focusing 1.5X magnified RF view with diopter correction. The Leica II became the mid-price model. A number of II bodies were later updated to III or IIIa specifications as well as earlier fixed lens I [model A] and non-standard I [model C] models that were converted to II specifications, like the camera above.

The plug at the back was a feature on "non-standard" interchangeable lens models and removed for proper alignment and adjustment of lenses to the body. This was necessary before the lens flange to film plane distance registration was standardized at 28.8mm. Although no longer necessary on the Leica II, Leitz used up all their remaining bodies during the initial run of the Leica II. The marking "O" at 12 o'clock on the lens flange from prewar Leicas indicate a standardized mount.
Initially available only in black enamel finish the following year chrome became available at extra cost. Between 1932-1948 about 52500 Leica IIs were produced, with a little less than half finished in chrome and the rest in black enamel. Despite the overwhelming number of black finish produced chrome models do not attract as much collector interest. The look of a "brassy" black finish with nickel fittings have more mystique. Times have changed, nowadays people pay a premium for "painted" versions of the M series and other vintage Leicas....

This is my user Leica II ca. 1932 serial no. 71605 acquired in virtually parts condition. Fortunately with experience gained from servicing Barnack type Feds and Zorkis, I was able to restore it to user condition. I never found the lack of slow speeds on the Leica II and FSU clones to be a major limitation for most outdoor or even indoor daylight shooting using moderately fast ISO speed films. Besides 1/20 is the threshold of my handholding skill and rarely get shots at that speed that would enlarge well to 8 x 10 prints. Although the separate and squinty RF and VF eyepieces need familarity in usage, the RF patch is life size for easier focusing and the 50mm VF is very close to the lens - parallax error is only about 2.5cm at 1 meter.
The most endearing quality of a Leica II or clone is its compact size. Pocketable with a collapsible Elmar 50/3.5 or the Industar clone lens with the flexibility of accepting other Leica screw mount type lenses. There is also a distinct superiority in quality with the way a Leitz body handles compared to mass produced FSU clones which is no doubt due to Teutonic engineering. However in the hands of a good photographer using the same lens, it is impossible to tell which picture was taken by a Leica or a clone.
Click here for restoration as well as internal pictures showing the similarity of Fed and Zorki clones.
EZiest way to spot a fake Leica II without handling the metal...

Pay close attention to the VF bezel
front to back - "notched" early Leica II, III and IIIa*, "rounded" late 30s II,III, IIIa and IIIb, "square" Fed or Zorki fake!
At some point I considered the purchase of a genuine Leica II counterfeit on eBay to complete my budget Barnack collection until I discovered that for not much more I can get a brassy black enamel Leica II that only needs a bit of TLC and CLA. Some of these Ukranian and Russian craftsmen have honed their craft to the point wherein their engraving is of such quality that it is difficult for the uninitiated to spot the fake. However the dead giveaway for a fake is the "square" VF bezel. All "pre-diecast" [before IIIc] Leica screw mount bodies had either the notched or rounded bezel as pictured above.
*Early Fed 1 and Fed 1a series also had notched bezels [initially without accessory shoe] but these models are so rare that they command even higher prices in the collector market only a fool will convert it to a Leica II fake. Visit this webpage for an example.

Stalin wanted a Leica....
FED and Zorki where the longest and most prolific manufacturers of Leica 39mm screw mount cameras and lenses.
FED
FED was named after the initials of Felix Edmundovich Dzerjinski, father of the Soviet Secret police later known as the KGB. It was a factory in Kharkov, Ukraine manned by young orphans of the revolution established in 1920 to fulfil Stalin's plan of turning the Soviet Union from a largely agricultural to a self-sufficient industrial nation. They started by copying electric drills from Black and Decker as well as other tools and products for industrialization. Soon after the Leica II camera was introduced the factory reverse engineered a unit and came up with the FED-1 equipped with a clone of the 50/3.5 Elmar lens. About 700,000 FED 1 and its variations were produced until 1955.
more information about the history of FED

Fed 1d [1938-41] - to my eyes and ears this is the best finished and sounding clone in my Fed 1 stable BUT it is a prewar model and the pitch of the lens mount will not accept any LTM lens [including KMZs] other than the one supplied with it. Lens registration is at 28.5mm, luckily it was not tampered and with the uncoated 50/3.5 lens took nice pictures.

Fed-S [1938-41] - Princelle states that this is a relatively rare unit with only about 2000 produced. It is essentially a 1c or 1d with a 1/1000 top speed. Notice how similar the vulcanite is on this unit compared to the 1d above. Here's a link to a Fed-S with original engraving.

With the top cover removed one can see the additional hole on the shutter speed disk for 1/1000 as well as the rectangular hole left of the "B" marking on the top plate which is a provision for a linkage to the slow speed escapement at the bottom. In the late 30s Fed succeeded in cloning the Leica IIIa and made about 40 prototype units with slow speeds known as the Fed-B. Since WWII was just around the corner, it can be speculated that the Soviets scrapped production plans the remaining inventory of parts were used for the Fed-S.

Summar clone 50/2 lens - standard issue on the Fed S and V. The lens registration on the Fed-S is the Leica 28.8mm standard and the lens mount will accept any LTM lens. However the distance and aperture scales or focusing tab ends up at an unusual location. The focusing tab on this Summar clone locked at infinity mounted on a Leica II ends up around 1 o'clock.

According to Princelle some Fed-S were engraved "Leica" at the factory. I presume this is one of those original fakes from Fed with a matching 50/2 lens that is also engraved "Summar". The finish on the RF cover in this particular unit was sanded down and then painted. Since the paint was flaking off I stripped it carefully and found an attempt to fix a dent near the accessory shoe.

Leitz engraving on a ca. '37 Leica IIIa, presumably the logo Fed was trying to imitate.

Fed 1f [1949-53] - this model has some qualities of pre-war models as well as the succeeding 1g. It retains the Leica type shutter ring guard. The lens registration is Leica standard as well as the thread pitch but the aperture and distance scales or focusing tab will end up in an unusual location. Similar issues as the Fed-S above.

Fed 1g [1953-55]- modern shutter speed progression [B, 1/25, 1/50, 1/100 1/250 and 1/500]. The shutter ring guard is replaced by a wide flaring collar and the shutter release is now threaded.

Fed engravings - [top to bottom], "FED" 1g 1953-55, "F.E Dzerjinsky" Fed 1f 1949-1951 and "Kombinat" Fed 1d 1939-1941
Approximate Fed 1 production serial numbers and engraving types.
As noted above all models prior to the 1g have issues either with lens flange to film focal plane distance registration [~ 28.5mm instead of the Leica 28.8mm standard], a lens mount thread pitch does not accept any other M39 LTM lens including FSUs as well as a lens mount that will accept an LTM lens but with the distance and aperture scale or focusing tab ending at an unusual postion. Fortunately all my pre 1g cameras came with their original lenses perfectly calibrated to the body. I took note of this for reference before performing any repairs.
If you are looking for a shooter and plan to use different focal length lenses, the Fed 1g is the only way to go otherwise go for a Zorki 1.
Zorki
Krasgonorskii Mechanicheskii Zavod - KMZ - produced optical instruments for the Soviet WWII effort. The factory was located outside of Moscow and was spared from the Nazi onslaught while the FED factory was devastated in Kharkov. As the FED factory was being rehabilitated in 1945, KMZ with the aid of Fed technicians and blueprints mobilized for the production of Leica II copies. The initial units were known as Fed/Zorki and it was not until 1948 when the official Zorki 1 was released; Zorki meaning "piercing eye". KMZ also became a benefactor of Zeiss Sonnar blueprints as a result of spoils of war reparations and produced the fine line of Jupiter lenses.

Zorki 1b [1950-51] - soft body with pebble grain vulcanite and has an almost "Leica like" sounding shutter.
SInce KMZ was also manufacturing the Jupiter line of lenses, particularly the Jupiter 9 85/2 which is a pretty hefty piece of glass, after the 1b all the bodies were die cast with reinforcement around the edges and eventually around the lens mount for greater rigidity and reliability in maintaining the proper film plane to lens flange registration.

Zorki-Zorki 1c [1951-53] - die cast body introduced, notice the "frame" around the body. Similar vulcanite to the 1b. The flash synch was a later addition.

Zorki 1d [1953-54] - die cast body with additional reinforcement around the lens mount.

Zorki-Zorki 1e [1954-56] - modern shutter speed progression [B, 1/25, 1/50, 1/100 1/250 and 1/500].

Zorki engraving [top to bottom] - Zorki-Zorki in both Cyrillic and Western characters, used from type 1b-1e primarily for export, "Zorki in Cyrillic stamped" Zorki 1d 1953-54 and "Zorki in Cyrillic engraved" Zorki 1b 1950-1951.
Approximate Zorki 1 production serial numbers
Interesting Zorki 1 with slow speeds!
I have yet to encounter lens registration or lens mount thread pitch aberration from Zorki 1s. KMZ got it right this time. Although the Zorki 1b has a "Leica like" sounding shutter the "soft body" precludes it from being a user camera. The 1c -1e models are the best users in my opinion.
Reference: The Authentic Guide to Russian and Soviet Cameras 2nd edition by Jean Loup Princelle available from Fedka.com