I had 2 '72 340 Dusters, both had drum.

The '72 window sticker has a high serial number.
It's possible (pure speculation) that late in the '72 production run there were more disc spindles in stock than drum. Maybe Chrysler was trying to use up the extra disc spindles by making them std on the late production '72 340 models. Both spindles were not needed for '73 production as the drum and disc spindles were different along with the upper A-arm.

edit;
RAII, you not doing yourself any favors with your sig line.