T. The original finish was varnish, also on light coat on the receiver.
And yes yours would be the same, FW S prefix. When Marlin took over all of the shotguns produced were FW S(Feather Weights) there were no more Regular models made. Regular models had only one screw holding the sideplates on, whereas the FW, FWS, and FWM, had 3 screws, 2 on one side and 1 on the other sideplate. The bottom protruding lug was also smaller on those models

David