Just wondering if anyone knows what causes certain discoloration on the backs of vintage cards that first appears as a "burn" or dark discoloration around the edges of cards. I have recently observed this on 1969 cards but have seen it on other vintage cards as well.

I have also occasionally noticed around the front edges of vintage cards with white borders as well. The discoloration does not seem to be the result of damage to the cards as I routinely see the discoloration on sharp cards in NRMT condition.