There are two long brackets that attach to the monitor tube bolts on each side horizontally, that stick up alongside the tube. These brackets have holes that the black monitor bezel screws into. There are 4 plastic "clamps" that the screws pass though that hold the tinted plexi in place.
This is used on many 80's games, not just the Pacs and Galaxians/Galagas. I know Spy Hunter used it as well....
As for pics.... I don't have any at the moment, but have some of these brackets at work and can get a pic of them if no one else has any....