This neat cup-like accessory is screwed into a shutter release button. Normally, it fits any button that can accept a cable release. It is approximately 15 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.
Made in Japan.
The image shows the button installed on a FED 1 and Kiev 4a (cameras are not included).