Update -- truck is a '75 6500 series. Not sure about Dayton or Budds. I'll ask about the numbers and measures. The owner of the truck went to the local garage (see below) and they came up with "Obsolete Part No Longer Available" so they went to the local NAPA. The NAPA owner came to me. I will pass the questions back up the chain and let you know what I come up with.
Not sure if the drums are actually unserviceable. Local big truck garage only experienced in high use commercial trucks and not really well versed with the realities of low-use/antique trucks. I'm guessing that when they pulled the drums and saw grooves deep enough to make the drums unserviceable if you were to grind them out, they said "oh, these are no good." As this is a farm truck that only comes out of the barn at harvest to haul corn or soybeans a few miles to a local feedmill ... my recommendation was that if the drums are still round with good thickness, they should put them back on and ignore the grooves. More actual braking surface anyway, right?
One of the wheels was locking up when braking which is why they went to the garage. They pulled the drums, saw shoes worn down to rivets and the grooves in the drums and the ASE auto-response kicked in.
I'll be back over there this week and will pass along your follow ups. In the meantime, they might have followed my recommendation.