I'll up the ante and say ift was part of a strap wrench rig. I've got an old Bonney rig like that with only one nub on the non-drive end, but I'm pretty sure it was for removing studs. Unless my perspective is off, the fully open position doesn't look like it would be wide enough for any belt ewxcept maybe a serpentine belt(and I'd think there would be a market for that these days).
Good thing guessing doesn't cost you brownie points on this forum, cuz I'd lose a lot of money playing Jeopardy!!
DM&FS