Originally there was a bracket on each side of the monitor tube, attached to the monitor frame at the same bolts that hold the tube to the frame. The bezel and tinted plexi were then screwed down onto the two brackets via 4 screws and 4 plastic clips.
Frequently when monitors were swapped out of these machines, the brackets were not transferred over from one monitor frame to another, leaving the original attachment points for the bezel missing. I've seen these bezels stuck on with tape, screwed to the closest hunk of wood, left floating around or flat out missing.
Wish I had one handy to show you a pic, no pacs in the shop right now..
D