I think I know what you're talking about. Rectangle shaped tang/peg that sticks into slide bore that keeps the slide from spinning? Not really a screw, it is kind of a press fit, then is "staked" in place. If you lost yours, maybe get a junk carb (stripped top, etc) take the tang out and put in yours. Use some super glue on it. After it dries stake it in place.
Staking is the process of deforming the metal around the hole. I used a small, sharp cold chisel and holding at an angle so as to only cut the edge of hole, staked it in three/four places, I forget how many I did. You don't need much of a hit as carb is alum.