S&W originally called them "plate screws" and they were differentiated by either large head or small head. The SCSW refers to it as the upper sideplate screw and the one on the .38 M&P airweight (pre model 12) was eliminated around 1955. Most of the parts lists that were shipped with revolvers were from after 1957 when S&W began model marking their guns and therefore, the sideplate screw is no longer shown since it was eliminated in 1955.
The remaining 3 sideplate screws at that time were referred to as the "small head plate screw, round head" and the "small head plate screw, flat head". As someone already stated, the rearward most screw was the flathead (under the stocks) and the lower and most forward screws were the round head ones.
As an FYI, the trigger guard screw is referred to as the "cylinder stop screw". Through the 40's and 50's these screws could be had for anywhere from .10 to .30 cents depending on which one you needed.
Some of the airweight guns also had a small screw next to the upper sideplate screw that kept it from rotating. Many folks refer to this screw as a "bug" screw, while others say that the "bug" screw was the larger upper screw. Personally, since it is small and "flys" around the larger upper sideplate screw, I agree with those that refer to the smaller screw as the "bug" screw. I hope that helps.
