Totally agree with hemi71x. If it had 9” drums all around it should be a SBP car. But in ‘74 a lot of the slant 6 cars got disks up front, and if they did they were 5x4.5”. The 4 wheel drums were standard, but there were several option packages that would trigger the front disk option. My Duster was originally a slant 6 car, it came with front disks and was BBP all around. 10x2.5” rear drums, although if you have 10x2.5” BBP drums then the upgrade to 11x2.5” is easy, just replace the backing plates and drums.
As for the 11” brakes being overkill, I disagree. Maybe on a 100% stock car, but it you run more tire out back you can use more brakes to go with. I upgraded my Challenger to 11x2.5” drums and did exactly the same thing with my Duster, I’ve had 11x2.5” drums since I swapped the rear axle over to an 8 3/4. They work great!
Sounds to me like someone swapped out those rear brakes at some point. So much for “all original parts”! Probably should check what axle is in there too. If the brakes went to 11”, there’s a good chance that wasn’t the only thing that was swapped.
