It seems the drum is indeed incorrect. If you a$$-u-me the rear is from a mid-seventies intermediate body, and the brake shoe size is 11 x 2 ¼ “ (taking into consideration the backing plates are correct) -
ENG. NO. D6OA-1126-BA should convert to PN D6OZ 1126-B (SEE PN LIST ATTACH).
D6OZ 1126-A is for 11 x 2 ½ “ shoes (WAGON). The bell depth for the 11 x 2 ½ “ is 3.580in.
D6OZ 1126-B is for 11 x 2 ¼ “ shoes. The bell depth for the 11 X 2 ¼ “ shoes is 3.560in. There is your 1/4” depth difference.
(Bell Depths Found On RAYBESTOS site)
Now all of this is from conjecture. One needs to know the backing plate I.D. and axle offset (donor vehicle info would be nice). I am using 1976 as this was the drum ID. I would think whoever did the changeover would have stayed in the same year(s).
RAYBESTOS has the drum(s). It (PREMIUM) is a quality part. You might want to find a take-off in a salvage yard to determine if this is correct before purchasing high dollar aftermarket drums.