The lens itself is very small. Unused SR-mount is glued to
49->55mm step up ring, then 55->40mm step down ring is
attached to mount this lens. So the lens is SR-mount now.
note: these two rings aren't glued.

You can use 42-55mm stepup ring for M42 mount system,
or any mount metal will be glued to xx-55mm step up ring
for a different camara mount system too.