This neat cup-like accessory is screwed into a shutter release button. Normally, it fits any button that can accept a cable release. The button is approximately 10 mm in diameter.
This accessory makes release button more comfortable to operate. Most shutter releases on Soviet camera have sharply machined edges, and the attachment eliminates this inconvenience.
Will it improve your pictures? Please try it for yourself if you are curious.
Important note - this button does not fit Kievs (their cable release receptacle is too deep for the button thread to engage). It works well on FEDs and Zorkis.
The image shows the button installed on the FED 1 and Zorki-4K (cameras are not included).