This is a quasi-scientific wild guess: These marks, typically as letters and numbers appeared on all the guns in my collection (targets as I define them, from beginning to end------the several NM #3's to the Model 52-2------the real beginning, what I deem to be the end). I decided the folks who had a hand in making these guns were paid by the piece---also that they were essentially employees of the several foremen (they in turn essentially independent contractors to S&W). Needless to say, any and all of this very likely changed over time as the government became more and more involved in the employer/employee relationship. I supposed payment for the services flowed from S&W to the several foremen to the workers, and these various markings served to determine daily production logs kept by the foremen which determined who got what.
Is that really the way it worked? I don't know----I don't even pretend to know. I believe it makes some sense----sort of.
Ralph Tremaine