The FW means it is on a Featherweight frame, and is now the only frame style made. The "S" is the new designation that the new L.C. Smith Gun Company, a subsidiary of Marlin Firearms Co. used when they started the serial numbers over again.
The Ideal Grade was the next lowest grade made. Some had nice looking wood and it was the lowest grade to have engraving. In a 20 gauge with ejectors, it is a nice gun and good luck with it.
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