Could also be an "IMPORT" - some machines had a plastic rather than glass - a sheet of glass was then placed in front of this. It prevented breakage during shipping.
If the machine has a 240V input with NO step down then this could be the reason - is the back sheet glass or acrylic?
My BANK A BALL has the same thing except clear sticky tape was used to keep the two together - this just crumbled off though - no issues at all removing it...
...OR.. could be as Tony suggests
Actually Gav,
There is another reason - if the import game had a plastic backglass the ops used to place a sheet of thin glass infront of the 'backglass' to stop the punters playing burning a small hole in the 'backglass' near the credit unit. The punter would then be able (through the small hole) be able to manually activate the credit unit. Then the game Dave has is from 1977 so they didn't make plastic backglasses for export then, only glass. Plastic backglasses were stopped being made around 1969 which is a shame as I have never seen a backglass on plastic that is at least a 9/10 from that time period
