Which vintage lens mount? M42?

Edit: I use this cheapass thing to adapt M42 lenses (Pentacon 135mm f/2.8 and Helios 44-2 58mm f/2) to my EOS R via Canon brand EF to RF adapter. It fits nice and snug and there have been no issues with focusing to infinity.

Should probably work just as well on a DSLR. The Amazon listing says "Infinity Focus Guaranteed."