the 1972 dealer code book shows power brakes & disc brakes to be OPTIONAL on Duster 340
but, it must have been confusing enough for someone to write "optional" on the copy I have

I have a b'cast sheet for a 11/24/71 - SPD, US spec. HAMTRAMCK Duster 340 - it is coded "B11" - drum brakes

here's a copy of a window sticker for a 12/12/71 - SPD, US spec. HAMTRAMCK Duster 340 - it states "power disc brakes" - STANDARD

IMO questions were probably raised why the disc brakes were no longer STANDARD & a change was made during the 1972 Duster 340 model year late-November or early-December (based on the few pieces of info in this thread)