Hey Wyo. Coyote Hunter,
That was a heck of a find and good on you!
I have a similar story, not quite as good but ...
I got a 25-lb keg of IMR-PB powder by being acquainted with my barber well enough for him to know I was a hand-loader of several calibers.
The guy in the chair asked if the barber would be interested in a can of gunpowder. (Its well known that he's a varmint [groundhog] hunter and loads his own.)
The barber's eyes met mine and he said, "probably not but that fellow there might," as he gestured to me. We both then asked in unison, "what kind of gun powder?"
The customer said he didn't know, he'd found this keg sitting in the trunk of an old car he was junking. Been sitting in there for unknown years. (At least it had been dry.)
He said he'd bring it by, "later."
Well he left, I got my haircut and I left.
A couple weeks later I called the barber to see about shooting some 'hogs, (an interest we shared,) and he told me that guy had brought back a "big keg" of powder.
He didn't know the type but had it in the closet for me.
Turns out its a keg of IMR-PB, lot number 6XX !! (Really old stuff I think.)
It had been opened but the cap was on tight and most of it was still there.
I made up a few loads of several different cast bullet loads, exact matches for the information I could find, (there isn't much for this powder,) and it gave performance very close to what the sources stated.
That was about 6 years ago. I now have more IMR-PB powder data than all info sources I could find put together, all developed here at home, (with a chronograph.)
I've grown to like it a lot and will probably be shooting it until I'm ready to quit.
Now if I could just stumble on to whatever the military surplus equivalent of Accurate #7 and obtain a bunch for cheap. Or maybe IMR-4895.
That big barrel of IMR-4831 is a seriously great find. Hard to beat that one.