Yep, I found a table of which rings fit which lenses on another site (comes up amongst the first results if you search "Canon CAT System").
Annoyingly the 35mm breechlock I've spotted is angled in such a way as to hide the pin. I can see the glass is good and the body looks tidy, but not that crucial couple of millimetres of the front end of the focusing ring!
It would have been logical for the pin to be dropped with the New FD lenses, certainly it would have made writing instruction manuals simpler.
Edit, a breechlock 50mm f1.4 just dropped into my lap. No scratches but needs a clean, according to the listing, and I could see the indexing pin for the flash ring. £25 including postage. That'll do!