It's a t-nut

Let me guess... A Gottlieb system 3. They put blue lock-tite on all the machine screw t-nuts. The only way to get them off without breaking the screw is to heat them up for about 30 seconds with a soldering iron.

You can get t-nuts from most hardware stores. Lowes, Home Depot etc.

Playfield screw holder. I love it, that is funny. Buy them online as they are very expensive if you buy them at a hardware store. They are 40-50 cents each at a hardware store verses 8-15 cents online the last time I bought them.

If those are from a gottlieb you may want to just heat them like uncivil_engineer suggests. They are probably 6-32 which my Lowes didn't have and the ones I found at ace had a larger diameter base and wouldn't fit in the countersunk hole on the bottom of the play field. Just hold the stub of the screw with some pliers and heat the nut to break down the loctite. A soldering iron will work but a propane torch will get it done in a few seconds. The smell of the loctite burning off isn't the greatest so you may want to do it outside.