We all know about the early mint set cardboard contributing to toning. What about the '68 and newer sets in the pilofilm packaging? How do you get toning in the package in the absence of pinholes? As Doug has mentioned all plastics are air permeable. This cut up 1976 set had a sticker or tape residue over the cent (still in the package) and I think over time released gas through the plastic. Hard to take pictures to show the purple hues but definitely there. The half dollar has some weird splotching as well. Feel free to add your mint set cut outs. Oh by the way nothing like feel of the sharp edges of a reeded half dollar freshly cut from a mint set.
Definitely different from album toning and not symmetrical or even coverage over the obverse or reverse or similar between sides as well.
