That mark on the barrel flat of your SBT is the mark of Joe Scaglione, the worker who proofed your gun. See pages 78 & 79 of Walter Snyder's book
The Ithaca Gun Company, From the Beginning. The 3 may be a choke marking for improved modified, but there is no choke marking on the three other Knick barrel flats I have saved pictures of. This gun was proofed by Charles Fox --
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On this gun Charles August put his mark on the lug next to the grade number --
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This gun was proofed by William Bean --
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