[Short 3/4]

My guess is that you have the original housing and someone replaced the entire brake assembly in the rear with the 13’s. It’s really easy to do, I actually have 11” drums on the rear of mine that I pulled off my old 2wd to get the truck running while I was looking for the correct parts. I now have the right stuff, but always have something better to do than replace parts that are working. Probably someone in your trucks past wanted bigger rear brakes, or more likely couldn’t find the 4x4 parts so went to the local wrecking yard and made something work. The 12” rear drums were only used on 4x4’s for 60-66, never 2wd, so parts were always uncommon. There is an interesting thread somewhere on this forum that talks about all the brake interchanges possible for the HO rear ends. As I remember you can even swap brakes from a Dana 60, so seeing the “wrong” parts on a 50+ year old truck is not so unusual.
Anyway, this is a very long winded way of saying you have a lot of options for drums if you can just manage to also find the corresponding back plate and hardware.
Hope this helps
Tom