Jim
The screw holds the firing pin in the upright position, so when the hammer swings up from below, it will not catch on the edges of the milled slot in the bottom of the firing pin.
It has a smaller diameter stud turned on the end, and there is a corresponding hole in the top of the firing pin. It is just a plunge cut with an end mill, but is larger diameter than the stud, and it's dimension is correct to allow clearance for the firing pin to move back and forward.
I hope this makes sense.
PM me if you need some dimensions.