I'm confused about my trucks rear axle

[ronh72c10]

I went to napa last week to get the parts I need to completely rebuild the brakes in my truck and couldn't find a matching part for the rear brakes on my dana 60. Basically napa says that there were two brake options for the rears 11" and 13" drums, my problem is that my drums have an internal diameter of 12-1/8". Way to small to be 13" and way to big to possibly be a worn 11" drum.

As I understand it 12" drums were put on the 67-70 rears, but poking around on Rockauto's website they show a 12" drum for leaf sprung dana axles for the 2wd 72 GMCs only not the Chevs. So now I'm completely confused.

It's possible that it's not the original rear, any thoughts on what I should be ordering for the rear brakes? Or how I might be able to better identify that axle?

Thanks,
Ron