Neat Hippie. I think you're right about my pic having the Yugo but the angle just not showing it well. The space between the x39 & M59 is just right for it to reside there. I'd like to use your photo as the official photo for that stamp if you don't mind.
As for when imported, certainly prior to 2015. The 2015 PW stamps were in the configuration of: PW ARMS REDMOND WA ZASTAVA M59/66 7.62 X 39 YUGO
I've got a later PW stamp that I just found that has different punctuation but the exact same layout, so I think that is a newer one (the 2015 from above came right out of the Classic unboxing when they first got these in in 2015, so I pretty much know it's pedigree)
I think I've got #20 and #21 in the wrong order. It makes sense that #20 came later as all the later stamps are also all
pin punched while #21 is engraved (and a bit roughly at that).
The ones with the Zastava are the ones's I'd assume to be the earliest stamps with the progression slowly moving to what is currently being stamped now. Yugos first came in en masse ~2001 I believe. Narrowing it down farther than that is a tough task I fear w/o being able to talk to someone at PW.
Good stuff Hippie, the Yugo boards don't get much lovin these days. Need some good Yugo examples to pique people's interest!