Nickfed is correct- Zorki and FED had lens mounts whose start threads were not always in the same place. Some of them had it so bad that the lens markings would be found at the bottom or at either side of the barrel when when the lens is fully mounted. This issue even affected other collapsible 50 lenses. It isn't at all strange that a similar collapsible 50 from another camera would mount with its tab right over one of the RF or even the VF window.
The older FED are more afflicted with this. The FED from the middle range (I found those with SN above 400XXX) to be more consistent. However the thread starts were still not in the same place, unlike the case of Leica or Canon. Many lenses still would mount with their index marks slightly to the left or right, instead of the centre. And this was never really resolved- even FED or Zorki from the 60s or 70s can have mounts which did this.
I would also venture to say that this is the reason why Soviet LTM 39 lenses always came with full barrel RF cams. With such cams, the lens can stop anywhere as it's mounted and still be able to couple correctly with the camera. Having the markings at the bottom is indeed an inconvenience, but if the camera's RF and lens flange register/working distance is correct, the lens can work well with the camera.
A FED in the 131xxx range may not work correctly with the later I-61. They are often found with non-standard (ie 28,8mm) flange to focal distances. Their RF camming is also different. The odd way the I-61 fits on it can be taken as an indicator of this. So unless a FED from this range had previously been adjusted
for a more recent lens, expect to have more problems than just having to read the scales from the bottom of the camera.
The good news is that you can always adjust the camera to suit your I-61, if this and later Soviet LTM are what you'll be using with it.
Jay
