i think the number you have on your drum may be a casting # instead of a part #. all the info i have on interchange numbers indicate the 69 charger R/T, coronet R/T, super bee, road runner[with drums], or belvedere, charger and coronet with heavy duty brakes or with trailer towing package had only 11x2 1/2 rear drums. that factory part # is : 2405789.[7 digits] if it indeed has 11x3 drums, it more and more leads me to believe they were swapped. the only way i believe possible to be sure is take the drum off and measure. i know that is not what you want to here, but before you spend the money i believe it's now the only way to be 100% positive. 11x2 1/2" stuff is pretty common [unless you want absolutely NOS parts] as it was used in trucks and vans for many years after the muscle stuff was gone, and is interchangeable. i have done it countless times. i can't say how many vendors chrysler used for their drums, so someone other than me will have to chime in for this info. let us know what you find out.

the alternate part # is 3004933. this is also an 11x2 1/2" drum. this is the drum with cooling fins used in 69-70 on all mentioned above plus fury, GTX, monaco, newport, new yorker, chrysler 300, polara, 70 barracuda, 70 challenger with heavy duty or trailer towing option, 70 cuda, and 70 challenger RT with drum brakes.