The Soviet Union started making copies of Leica cameras in the 1930s. Patent law was on again/off again in the USSR, mainly off while the Communists were in power. Inventions were state property and they were in a cold war with the rest of the world, so why honor other countries' patents? The Soviets reverse engineered Leica and other companies' cameras. Then after World War II, the Allied powers basically negated German patents and anyone was free to copy their inventions. Hence the Reid, Nicca, Leotax, Tanack, Yashica YE and even early Canon rangefinders.
The Soviet copies were originally branded FED or Zorki but enterprising forgers have replaced, ground down and/or repainted the top plates and re-engraved the cameras to make them look like Leicas. The most obvious fakes are Leica II looking cameras with big Nazi Swastikas engraved on the top and exotic coverings like highly polished wood or snakeskin. Actual Leicas used by the German military in WWII generally were much more understated and didn't have swastikas.
My Leica I copy had an over the top wood covering on it which I replaced with traditional black leather. Another clue that it is a fake is the 0275 serial number on it. A Leica with that serial number would be from the first batch of production models made in 1925 and have the infinity lock "hockey stick" on the front (although often these were removed during later upgrades to make them Leica Standards). A real Leica with that serial number would never be in this great shape with a thick lacquer coating over the black paint, shiny chrome screws, etc. And the cold shoe would have had a rounded base. The engravings on the knobs on this one weren't filled in with paint when I got it, but in the spirit of making this really look like a Leica I, I filled them in. The viewfinder on this copy has more rounded edges than the actual Leica I had.
As to the quality of the camera and lens, the infinity focus is off on the Industar-10 lens (which is actually a copy of a Zeiss lens, not the Leica Elmar that would have come with a Leica). I tried to take it apart and adjust it, but couldn't make it work. So I have to not extend the barrel all the way out and have an ink mark as to where infinity approximately is from testing on a digital camera. Also, because the screws that hold the body to the internal mechanisms were removed when they put the wood covering on it, the guts move around and make it difficult to load a roll of film into it, but if you work at it, you can get the film in and then need to use a needle nosed pliers to get it back out.
Here's a side by side of this fake Leica I with an actual Leica Ia from 1930 which was converted to Leica Standard (many people did this in the 1930's to be able to use interchangeable lenses).