Richard, Welcome. Your gun was made in 1949. The FW means Featherweight and after Marlin bought out Hunter Arms, the new guns made were only FW frames and the serial numbers started over again starting with FW S1. So in dating this period of guns the "S" is the first letter of the serial number. As for not stating on the barrels Hunter Arms, now you know why.
I have a 1946, first year of production for L.C. Smith Gun Co. under Marlin, serial number S1829 that doesn't have any top barrel markings at all.
The lettering on the bottom of the barrels watertable are the proof marks that Hunter, and Marlin used. It is a symbol of HAC (Hunter Arms Co.) with NP Nitro Proofed) incorporated in it. Also when the chambers were lengthend in the 20 ga. from 2 1/2" to 2 3/4" by the late 1930's they stamped the chamber length. The 16 ga. was changed also in the late 1930's as I have a 1939 that still has 2 9/16" chambers. Some have stated that by the late 20's some 16 ga. were chambered for 2 3/4".
Hope this has helped you and please do become a member, as it is very addictive.