I just purchased a red leitz 36mm filter on ebay. I thought it would fit my industar-50-2 (m42 mount), but apparently it does not. What lenses were these made for? I am planning to get a Fed 2 and a fed 50/3.5, will that fit it?
A good question. I am not sure about the lenses for your FED 2. There is also the point that one of the standard filter sizes is 35.5mm, which is very close.
My Canon III has a collapsible Industar 50 attached. The yellow filter I am using is a push-on Ernst Leitz with a knurled locking nut on the edge. The inside collar measures 36mm.