Without trying to start a war, let me pass on some things I've learned, and I believe to be true. Clearly I wasn't in the Soviet Union when these things were occurring, so I can't be totally certain, but the details of the lenses match the story I've heard.
First, the Soviet knock offs were superb, and I have a few. I cannot afford a REAL Leica, and I promise you that in my hands not a person on this board could tell when I was using a real Leica, my knock offs, or my old Yashica. I'm just not good enough that the build quality of the stuff, so long as it's not plastic Holgas, matters. In my case the photographer is the limiting factor, not the equipment.
But the arguments come up all the time about Leicas and LTM mount FEDs, Zorkis, etc.
First, the Soviet lenses were very good glass, and build quality of the shipped lenses varied from superb to so-so. This differs from Leica build quality. At Leica, if it got out the door, it was right. But if you get the Russian lenses serviced properly they're excellent. This is an artifact, so I'm told, of the fact that the lens craftsmanship was tied to the Soviet satellite imaging, and they spend money on it to make it good glass.
Second, the Soviet LTM lenses will screw in to a real Leica perfectly, but they don't focus properly without adjustment. Likewise, a perfectly adjusted FED will accept a real Leica lens, but it won't focus properly. And this is the tricky part.
The Soviets usurped two sets of lens technology after the war, Contax and Leica. But they got far more parts and manufacturing machinery for Contax. The Soviet LTM lenses use a Contax helix pitch standard (instead of the Leica helix pitch) as a way to consolidate manufacturing and reduce costs. The two are close enough that you can fit them into the lens, but they are *NOT* the same. Ergo, the rangefinder cam on a FED or Zorki is shaped to match a Contax helix pitch instead of a Leica helix pitch. Consequently, while they screw together, the rangefinder doesn't work right if you mix the Leica and Industar lens. (Unless, of course, you have the lens modified to work with your Leica.) So everyone complains - unfairly - that the Soviet lenses are junk.
Like I said, some details may be wrong. I wasn't there. But this is the basic story I got from a Ukrainian guy.
MB