Cool! I hope you managed to preserve it, those are often quite rare. I collect paper milk caps as well as bottles because a lot of local dairies didn't use their own bottles (and those that did are usually out of my price range) and I never imagined it being possible to find one underground, though it makes sense considering the newspapers I see dug. I found a foil cap with my metal detector once, but unfortunately it was when I was on a trip down to the US so it wasn't a local one for me.