I've never noticed date stamps on those, but then again, I never looked for them either.
Here is what I have from my notes.
"Prior to 1971 Klipsch used surplus PIO types (decades old) that had glass hermetic seals, these type seldom fail. After a short (failed) experiment with Mylar types new PIO types were obtained, but the seals do not look to be glass hermetic, and they fail (after a couple of decades of use)." -- DJK
"The older oil caps were WWII military surplus, and were hermetically sealed. Those have a much better life than the ones Klipsch had custom made for them in the mid 70s (and later). I've seen PIO WE oil caps from the 30s still be OK, they're hermetic with glass seals too. PIO Vitamin Q caps are hermetic with glass seals." -- DJK
"Prior to 1971 PWK used whatever cheap surplus oil caps he could get his hands on. After that stock was exhausted he went to some cheap Mylar caps. Those were so dreadful he had new oil caps made. Later he moved back to Mylar." -- DJK
DJK (Dennis) used to be a Klipsch dealer. I'm looping in @Marvel on the question of whether Dennis knew PK or not. I just don't remember.
Whether in an old post or during a phone call, Dennis told me PK had barrels filled with parts on the factory floor, and employees would dig for what they needed. Some built crossovers at home for extra money. He told me all of the capacitors were from surplus left over from the war. "Surplus" would be NOS in this context - not old used stuff.