FWIW I checked a number of 1969 Lynch Rd. Road Runner broadcast sheets:

all RM21s with M2_ DO NOT have C23 (rear arm rests) coded
all RM21s with M6_ have A87 (décor package), C55 (bucket seats) & C23 coded
all RM21s with H2_ have A87 & C23 coded

all RM23s have C23 coded

I also show RM21 A12 cars with the base M2_ interior built at least into the first week of June