Rick, yes the FW stamp means Featherweight. You will also encounter "FWS" prefix on L.C. Smith Gun Company guns made after Marlin purchase the assets from Hunter Arms in 1945. This frame was used in 12,16, and 20 ga. guns. All FWS Marlin era guns were made on the Featherweight frame.Quickly identifiable differences:
Added screw in each sideplate, under the breechface.
Shorter distance from breeface to hinge pin, as well as shorter recoil lug showing through the bottom of the frame.