This turns out to be a “Petri Bayonet” mount, as I suspected proprietary to Petri. I can’t find a Petri Bayonet to Canon EOS adapter, but there is a Petri Bayonet to M42 adapter for $30 that should work, since I can just pair it with my M42/EOS adapter. I’ll have to find another Petri lens I want to use, though. $30 is a bit steep just to try out one lens. I will probably spring for it if I find another Petri lens, though.
Mamiya/Secor 1000 TL, a late 60’s model. seems to be well-regarded.