Trim Stab.
I read a good book some years ago about a guy doing selection for 21 SAS (Southern TA). It follows this guy right through from first training evening to the "badging" ceremony.
It's called "The Quite Soldier" by Adam Ballinger. Well worth a read.
My original point was that whatever the actual course (P-Coy, SAS, RMs, Womens Auxiliary Balloon Corps...take yer pick) To have got through such hell and wear the badge (whichever) is a real achievement.
The "special" descriptive is for a reason.