The oldest Elgin 18 size pocketwatches have just one case screw holding the movement in the case.
Opposite this screw there is, or should be, a little brass pin in the edge of the movement that goes into a little hole in the inside edge of the case. One advantage of this design is that the case can have two hole so that either an open face, or a hunter case movement can sit in the correct orientation.
A disadvantage is of course that this arrangement is not as secure as one would like. And the pin is often missing, as it is on this example.
As a fix for this, rather than make a new pin, someone has replaced one of the barrel bridge screws with a longer one, and added a handmade brass washer large enough to go over the edge of the case. But I'll replace the pin instead.