Roger your serial number dates it to 1920. At this time there were both the FW (Featherweight frame) and the R (Regular frame) guns. The Featherweight guns like stated have 2 screws on one side of the lock plates and 1 on the other side, whereas the Regular frame only has 1 screw on the one side and none on the other. The lug that protrudes through the receiver is also longer on the R frame, 1 3/16" long x 1/2" wide. On the Featherweight the lug is 11/16" long and 3/8" wide. I believe Gunparts (Numrich) that they only have the FW parts. I have a 1946 L.C. Smith FW S1825 that doesn't have any markings on the top of the barrels only has L.C. Smith stamped into the side plates Also when the S prefix started they only made Featherweights.