Flues grade 1
It does appear to be a grade 1 Flues model (1908-1926) produced in 1920; 315400-343335. Page 352 shows boring for guns and says that before September of 1931 28 gauges were 2-1/2" and after they were 2-3/4". The S may indicate Special which could include special stock length, special stock drop or special boring. This was an additional $7.75 option available.
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