On some machines (Ballys from the late 70's and early 80's) the bar that
locks the backglass in place has sharp, 90 degree edges. When in the
'unlocked' position, this edge will point forward and upon removal of the
backglass can scratch the paint in the upper left hand corner of the
backglass. What I have done to fix this is remove the locking bar and grind
off those 90 degree corners to 'round'.