You have the "mushroom" style strap lugs presumably. Measure the diameter of the narrow part of the post. Some cameras use strap ends that fit over a 4mm shank, and some fit a narrower shank closer to 3mm.
4mm: Bronica S2 and EC, Mamiya 645 original series, Fuji G690, Pentax 6x7 (*)
3mm: Hasselblad (*), Bronica ETR (*), Kiev (*)
Ones marked with (*) I didn't measure personally, but got the information from an Optech strap website. Anyway, this suggests that the ETR strap should not fit an S2.