WADR, I've always known the "Three Ts" to be Target Stocks, Target Hammer and Target Trigger and assumed everyone else understood that too

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That is the way they appear on the top row of all the labels on my two piece blue boxes; to the right of "Model No.", "Fin" as in finish and "Barrel" as in length is a box labeled "Stock" where one might find written in an "R", "ST", "RB", etc. or, yep a "T" for Target then a "TH" box as in Target Hammer followed by a "TT" box as in Target Trigger, each of which would be checked or left blank. There are the "Three T's" in a row Target Stocks, Target Hammer and Target Trigger, as least that's the way it makes sense to me.
It makes even more sense on second glance, since by Model and Engineering Change (dash) Number one would know if it is has a "Click Adjustable Micrometer" rear sight in virtually all cases. If not, I would think that special feature would be addressed in the "Special Features" box in the middle row of the label.
Sight options are not listed until the third row, to the right of the SN where there is a box labeled "Front Sight" where you might find "RR", "P", etc. followed by a box labeled "WO" for the White Outline option on the Micrometer rear sight.
Besides, the SCSW says
"Three T's: Shorthand for target hammer, target trigger, and target stocks."
Good enough for me, but I'll be sure to make sure in the future that whomever I'm dealing with is on the same sheet of music, just in case

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