Box label shows S for stock. What stock does that represent??

“S” = standard, “T” = Target.

The gun came with Magna stocks.

Edit to add:
Some models were offered with round or square butt grip frames (Models 10, 34, 36, etc…). In those cases, the label would be marked “R” for round and “S” for square. Round butt target stocks didn’t exist in the 60’s and 70’s. This doesn’t apply to the OP’s gun, since the 17-3 was only offered with a square butt.

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thank you sir