Russar-Apochromat 2.3/50mm lens
This is special apochromat lens. It features no aberrations in visible and IR spectrum. So, you can focus it in visible light, and shoot on IR-sensitive film them without necessity to correct focus. The lens has extended flange focal distance (70mm from rear element to focal plane), so it can be mounted onto virtually any 35mm movie camera. Also, it covers 35mm photo frame size, so it can be mounted onto 35mm still image cameras too. It has stopper of the focusing ring.
We don’t know it’s current mount.