Goes in the lightbox, something to do with the lock or lock levers or... something like that, not sure. Looking at the angle, maybe it slides up against the strike plate(s) ?

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According to the 1978 Gottlieb parts catalog P, part A-15481 is called a glass wedge and belongs in the lightbox of 2- and 4-player games.

Wish I could provide a photo, but from a quick inspection of my only Gottlieb EM multiplayer (from 1973), I could not spot this part.

Couldn't see them in my '75 Fast Draw either. Look for a couple of empty screw holes in your lightbox to see where they may have been. Perhaps they are a temporary part normally removed after shipping?
There are thick metal pieces on my FD in the bottom corners that hold the backglass from shifting side to side, but rectangular and two screw holes.

They would be placed on one side of the backglass low in the back box.

One would be positioned with the long edge to the side of the backbox with the taper pointing up and secured with a screw.

Then you would place the backglass in position. The expectation would be that there would still be a gap between the wedge and the backglass.

The second wedge is then inserted rotated 180° from the first one as a wedge to fill the gap between the first wedge and the backglass. Not jammed, just close. Then secure the position of the second wedge with a screw.

If these were removed from your game there will be signs of the screw holes on one side or the other of the backbox.

Wonder when they switched from using rectangular ones on both sides to these on one side. Or vice versa?