I've never found the answer to this question either.
Just guessing, it could be a lot of things. It could be indicating that they tested the hardness of the steel on the slide, it could be indicating that they tested some other aspect of the pistol, and/or it could be indicating who at the factory tested that pistol, or what batch that pistol was a part of.
Or, to put it shortly, I don't know.
Just guessing, it could be a lot of things. It could be indicating that they tested the hardness of the steel on the slide, it could be indicating that they tested some other aspect of the pistol, and/or it could be indicating who at the factory tested that pistol, or what batch that pistol was a part of.
Or, to put it shortly, I don't know.