That bar mechanism has damaged many a classic Stern backglass. You might want to simply leave it off. Unless you are placing the game on location, this bar isn't really needed when the game is just sitting in your game lineup.

The locking piece holds the hooked piece up against the backglass as it holds the glass in place. Eventually you will see a thin line of clear glass start to form where the bar has worn through the backing and ink, probably due to nudging while playing. My guess is that even more damage is done when the game is being moved around from location to location than when it is just sitting in place. While SEI was in the backbox copying Bally's boards, they should have copied their backglass lock mechanism, too.