Usually the Faithful get a piece of blessed chalk "to go" and do the actual marking themselves when they get home.
The letters stand for the names of the Three Kings.
I just want to add something important:
Traditional Catholics should restrain their feelings of frustration when they learn about practices, devotions, chant, etc. that "their Trad chapel didn't do". I grew up at an independent chapel -- if it wasn't Sede, it was indistinguishable from Sede -- and there were countless things I FIRST experienced/saw at the SSPX seminary and/or SSPX chapels once I got into that milieu.
I can relate to that frustration. A little voice that screams, "AAUUUGH! I'm a lifelong Trad and I feel like such an idiot for not knowing this!!! When was someone going to tell me about it? Why were my priest(s) so delinquent in this regard!?!"
Long story short: lifeboat "Trad chapels" can't preserve EVERYTHING of Catholic practices and culture, although the best of them at least try their hardest. Talking about customs, hymns, devotions, you name it.
For the most extreme version, look at the Hidden Christians of Japan. They were probably missing some serious doctrines and practices, the equivalent of cutting into the meat (not just the non-essentials or "fat").
Moral of the story/Lesson for today: Maybe we should read to our children various books about Catholic customs, expose them to more chant/artwork/statues that they don't get "every Sunday", and other like activities. We basically need to fill the void left by the Crisis, to keep knowledge of the FULL PACKAGE of Catholicism alive and well.