Interesting letter, thanks for posting it.
I have to say, I thought a single letter S meant "Standard", that is, standard magnas not targets. Furthermore I thought pretty much all N-Frames were square butts at that time, so the designation "Square" would be redundant / unnecessary. Did Factory-Made Round Butt Targets even exist back in the day?
Well, it wouldn't be the first time I was wrong!
Can someone confirm that S&W box labels often have the "Stocks" area marked "TS" for "Target Stocks" instead of the single letter T?