I've several mid '70s Gottliebs, and have noticed lately that there is some
fading on a few of the backglasses; not 'general' fading, but small random
squares, about an inch. Upon investigation, the cause was found to be the
small black rubber pieces on the lightboard, that press on the backside of
the glass. I opened the board (Like when you replace a light in the
backbox), and it was stuck to the glass (again!), but this time the rubber
separated, some of it sticking to the glass. These machines only see
'seasonal' use, and during winter the room temperature gets down to the 50s.
I recommend removing these randomly installed rubber pieces, and use foam
weatherstripping, with the exposed side having a clear plastic coating, so
the foam-rubber doesn't touch the glass directly, just around the perimeter
of the lightboard; that should be enough support for the tempered glass
(1970s and newer) in normal places.