dag.odenhallyeah, the add-on is the glow in the dark. TEC sells the DIY kit with 2 clear tubes and minor sundry items, user sources their own trit vial (easy enough to do, can even be found already installed in the clear tube used in the DIY kit, just takes a bit of a wait for shipping)
I have several of this style, Ti & aluminum, but I've replaced the GiTD pellets with tiritium, since when I need to find my keys it's usually dark and they do not retain THAT much glow. HI HI