I've found a Zorki 4 with a normal M39/LTM mount and an extra outer bayonet. It looks like this:

On the back there is a normal serial number (5852462) and a other number on the bottom plate upsidedown (58073):

Do anyone knows for what this camera was used resp. in what kind of set it could be found? Thank you for any idea in advance.
Best regards - Guido