James, your gun is a transition gun from Hunter Arms Co. to the L.C. Smith Gun Co. When Marlin took over in 1945 they continued to assemble guns that were in the process of being made, and kept the serial numbers in sequence. Starting in 1946 guns produced by Marlin were prefixed with an "S" as is FW S1-. When Marlin re-introduced the L.C. Smith in 1968 the prefilx was FW M, and they were all Featherweights. As far as the sideplates being blued, I have heard of others, and like said anything is possible. Since the care-hardening is approximately .002-.004 deep, you could take a very fine tooth file and on the back side of the sideplate see if the file skips over this area. If it starts to dig in then it is not case-hardened.