I just tried the Optech strap with the B clip; nope, don't fit. Gonna have to send it back.
For whatever bizarre reason I discovered I am the proud owner of a pair of hand straps for the Pro TL, neither of which I am ever going to use, so I can cannibalize the clips for use on something else. (My apologies to any cannibals out there who may be offended...) Now to identify that 'something else'.
I have a dozen or so assorted Optech straps that I'm pretty satisfied using with 35mm cameras, but the Mamiya is a heavy beast and those quick release buckles look suddenly very flimsy. So I'm looking for a strap that is heavily padded and does not have quick release buckles. The only one I've found so far is the Crumpler Industry Disgrace. Any other suggestions?