Unidentified mark on a hammer

[painter777]

S is Standard Products...... It's a Milled Type III Housing (SQ rear Lug Top)
Stan Pro used a number system that increased as their production went forward, That's where the 13-A comes in to play.
Thanks to a over worked fellow member you can refer to this link and compare Hammer Types:

The U.S. Caliber .30 Carbines - Trigger Housing Group

BTW When you do get around to taking the Housing apart, Remember the Hammer Spring goes in from left to right.
The link above will show you why.

A side note: Not sure where Kingsport is ? I have family in Nashville, Brentwood and Johnson City.

Later.....

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[USGI]

Remember the Hammer Spring goes in from left to right.

Charlie, we know you meant Hammer Pin, not Spring. I've made some posts saying hammer when I meant trigger, so don't feel like the Lone Ranger! - Bob