Larry, all the L.C. Smiths I have seen with marking as to chamber length have been on the barrels water flats below the serial number.
This one is from 1949


On earlier 20 gauge and 16's that have no markings on the flats and are 2 3/4" chambers, were done outside the factory and so not stamped as so. Earlier guns had no stampings as to chamber length except some very early Fulton 10 gauges with Chambers 3 Inches in an oval and 12 gauge Longrange/Wildfowl guns with Chambers 3 Inches in an oval. These are the norm, and there were others so marked that were done by the factory.

David