On Bally backglasses you will sometimes see paint missing in the upper right
or left corners. This is due to the 90-degree metal edge of the locking bar
in the backbox. In the 'open' position, this protrudes past the wooden
guides and will scratch the backglass as it slides up along those wooden
guides into position. I remove the assembly completely, go to the grinding
wheel and round those 90-degree corners. Might save a backglass or future
replacement glass from damage during a hasty insertion.