I believe it is because cars with front drum brakes have 11” rear drums, which are finned. Whereas cars with front discs have 10” rear drums, which are not finned.
Because finned are stronger & less likely to warp, unlike earlier which were not finned & had those awful coil springs. Same reason they started putting fins on the inside of rotors
When we turned drums back in the day there was a “device” wrapped around the drum to stop the chatter. Same idea as the spring. I also have 11” drums finned and no finned so the size doesn’t mean finned. Chrysler did make a few front drums finned and back ones not.
Advance, I also checked Auto Zone and the others but they had Canada or USA at advance it is luck of the draw as to what they have in stock Also, Check Rock Auto.