Humpalot:

The backglass has flat-black (spraypaint?) applied to the entire backing (except the score holes).

I think this was done to reduce the light shining onto the playfield glass by previous owner????

What you see in the picture is where some of the black paint has chipped off, and allowing light to shine thru the original paintwork.
The original paintwork appears to all be in tact, and i look forward to the rest of the black backing falling off ASAP. 

MM.