Kelly,
I have seen a few rears like this. Two were early Chevy II, one was for 67 camaro, and one was for 68/69 camaro.
Two of the rears had later revised center castings.
The 67 camaro housing was a "341NF" center and one of the Chevy II rears was a "860NF".
I would assume that GM made them for other cars.
I sold the 68/69 rear for $2000, in the original crate.
The guy with the Chevy II rear is asking more than twice that for his.
But beware of "retubed" rears
, make sure the rear is real before paying for it. I have seen several people get burnt with "retubed" and restamped rears. They are easy to spot, if you know what to look for.
I hope this helps.
Eric