It's a bingo - pretty sure like okorange says it's one of the flat sockets with the single 'tooth' that pokes the nipple of the bulb. They can break if the tooth is adjusted.

Regardless, Pinball Resource is worth a call. He may have some. If not, you'll have to buy one from one of the parts guys (like Joe Shope) or pull from a parts machine. I've not seen any new sockets, so if Steve has some, please let us know.

The ones the bite into the bulb will do that to all bulbs eventually. You can press the tooth from the back and it will bite a little harder, but you can break the tooth. Alternatively, you can put a small blob of solder on the bulb nipple. It can push the tooth further out depending on size of the blob.