I'm an idiot; I think I know what it is now. I was trying to read the numbers and google 'USPCC ###' when what I should have been doing is thinking about what it's likely to be and then checking the number of that stock.
I guess I
ate my words about being able to figure this stuff out; and it was completely staring me in the face the whole time,
too. Oh, well, I've worked it out now and I guess all that remains now is to give myself a nice big congratulatory high
five.