The S in the serial # usually means it was built by Marlin after the war. Feather weights usually have the prefix FW in the serial number and left lock plate has 2 screws and the right plate has 1 screw. I hope this helps. The fore end iron should have the same # as the frame and barrel. Both of my post war feather weights have LC Smith stamped on the lock plates and a stamping on the barrel LC Smith Field etc