Several years ago, I had a 1973 M42 Mir 1A lens (2.8/37mm). I saw very little documents about it.
Optically, it seemed similar to any other Mir 1 but the aperture system was odd: only one ring to turn with a chrome button and a spring. To adjust the diaphragm, I needed to push the button and turn the ring. As soon as I released the button, the diaphragm was linked to the ring which was used as any preset lens (full aperture to preset aperture by turning it). If I wanted to go back to full aperture, I just had to push the button and the diaphragm was set to nominal aperture.
Now, the question: knowing that preset Mir 1 lenses were made since years, what was the specific use of such lens (if any)?
BTW, there is one to sell on Ebay (Item #360076906260): not really cheap ($80) for a Mir 1 but it is not the usual type...