Quoted from Pinball_Muggle:
don't drill those on the backglass frames - the type that unscrew - not made anymore ..drill away on the back & front door ones.
Hey, thanks, you guys all helped me figure out how to do the least damage. This is what I decided is the "correct" procedure to get into a locked bingo:
1) Check for keys taped/screwed to outside of head or cabinet
2) Try other similar keys
3) If front door is locked, drill it first and look for other keys inside on hook or loose
4) If no keys for head siderails or back door found, drill back door locks first
5) Pry caged nuts off of one or both head siderail locks and remove locks undamaged
I still have to repeat this procedure 13 more times to get through the pile of mostly 6-cards I got from Lloyd at Coinopwarehouse...the first two I opened are now both working about 90-95%, Nashville (backglass key was inside coin door) and Wall Street...they all look complete and have balls and good backglasses and playfield glass. No real "gems", but all look like they're from a bunch that was put away in working condition.
Got plenty to work on for a few years...