1966 SWB Step Rear Brake ID

[Casey66]

Just finished up replacing rotors, calipers, lines etc on the front brakes which a PO had upgraded to 70's era 5-lug disks. Got parts for a '77 and they worked just fine.

Now I'm getting started on the drums in the rear which look like they've also been swapped at some point to 5-lugs. I don't know if the PO used parts from the same source as the front or not, so I'm looking for advice on what parts I need to be looking for to do a rebuild on the hardware.

The markings on the front of the drums are:
3989541
MAX DIA 11.09

A search for that number pulls up an eBay listing for 67-72 CHEVY TRUCK NOS Rear Brake Drums 3989541. Since that's the only result, besides a posting here about mismatching drums on a '71, I wanted to see if anyone has any further info on these.

Here are some pictures:


5-lug drum on rear-end.


Markings on 5-lug drum on rear-end, "3989541". Also has marked, "MAX DIA 11.09"


5-lug drum heat side heat sinks on rear-end.


Inner plate on rear drums.


Upper inner plate on rear drums.


Lower inner plate on rear drums.


Brake pads in rear drums.


Inner hardware on rear drums (1).


Inner hardware on rear drums (2).

Thanks.