All 68-9 Firebird rear axles I have seen have the same large impression in the top of the axle tube just above the spring perch area as shown in the picture above. This appears to be to give the rear end more movement before it hit the rubber stops mounted on the frame rail. The 67 Firebird rears do not have this impression. The center section also has the round mounts for the bushings to mount the upper control arms used with a coil spring application. These are left empty on the Firebird applications. I do not see these in the pictures so either they were cut off or the center cast section is from another application. If the carrier has a 12 bolt ring gear then that is what I would suspect.
The standard Firebird used the smaller 4 bolt pattern to mount the backing plate, same as the Camaro. My guess is that is why they fabricated their own mounts. Since the standard Firebird used the retainer style axle bearings the axle tube does not have the same "dip" as seen in the standard Camaro rear ends. The "service package" JL-8 rear had the larger diameter axle bearings and thus did not have the "dip" near the end of the tube
. Both production and service package Camaro JL-8 rears utilized the larger bolt patten because they were adapting the Corvette brakes. Measurements from Bruce will tell us more.