FSU accessories--The Soviets made all types of accessories for their cameras. Very few are seen here in the US probably because they are low ticket items. In the mid-90's I weekly frequented a Commission Store in Almaty where they sold cameras and also had filters, tripods, flashes, lightmeters, timers which screwed into the shutter release-both spring and pneumatic types. (Unfortunately this store is long gone.)
Stereo adapters were also available but rare.
A look at Princelle tome on Soviet cameras has a brief description of some of these.
My favorite is a small tripod- a Leitz copy which works well with a very small ballhead like the Bogen 3009.
Popular sizes for filters were 40.5 and 49mm-- the 40.5 for the 35, 50, and 135 lenses. the 49mm was for the 85mm lens.
Slip on filters were made for the earlier Industar -22 lenses
Rarer filters were made to fit the Iskra and the Moskva 4 (the same size)
The yellow (jolti- Russian for yellow) Ж (J in English) filters and the orange really make clouds pop on landscapes.
We must remember that the Soviet camera and photography industry was second to that of Japan and perhaps in first place for a short while.