Trying to figure out my brakes

I'm going to vote with Redfish on this one. Prop valve and reversed rear shoes.

I checked the parts manual and found that the factory fitted the same power brake booster to disk cars as it did drum cars. There is a second part number that was designated for field conversion of manual to power assist cars.

There are three potential master cylinders for 1972. 1 each for disk brake cars, 9" drums, and 10" drums. The power brake cars will have a longer pushrod to the master cylinder. The disk brake master cylinder has a smaller forward well, than the rear. The drum brake M/C will have equal size fluid wells front and rear.

There was also a running change made to the master cylinder pushrod on 1 March 72. Hope this helps.