Exhaust split collar?

Looks like you need 78-79 style collars, look at the length of the collars in Ed's picture compared to the length of what you have in your pics
and you said it had been on a 79 650, it also would match up to the rust marks on the pipe. I believe the O.D. of the flange on the split collars are the same as later models. But I'd check as TexasKZ noted

The exhaust holders are the same on the 650s & 750-4s. Kerker half-moon split collars are probably going to be hard to find. I did not see any in the Supertrapp catalog (I'm assuming supertrapp now owns kerker). I would get the earlier 650 exhaust collars, then do a test fit with used gaskets. If the fit seems okay, then get new stock type gaskets or whatever you prefer. If not, get thicker. Z1Enterprises.com has fiber gaskets that are about the same dimensions as the copper crush gaskets, but thicker.

note: the needed exhaust collars are still available at the dealer as they were used on some late model ATVs. see pic at partzilla.