I think its caused by the black sponge they used to put behind the glass to cushion it from vibration against the back board when the game is moved.
I've seen it heaps on Gottliebs especially.. something about the sponge, over time leeches through the ink and leave marks that show up from the front.
Pic from a Card Whiz back board i have here in pieces atm..
+1
Often around the credit unit as well as the credit unit was mounted on springs and rubber grommets and vibrated a lot.
