Just a tritium holder, VERY robust and good looking. This is based off the design of the Glowworm from Berlin TiWorks, a just downright beautiful set of designs he has made, this was for those of us who couldnt afford the 120 dollar machined titanium fob, but still wanted that equipment. I love mine, i wear it every day, i have hit it accidentally with quite a bit of force, as well as dropped it a few times, yet the tritium is safe inside! If you want to print it, go for it, the price isnt marked up like other designs, if you dont beleive me, you can download my design from thingiverse, and upload yourself. Tritium can be sourced from various sellers, i would recomend mixglo for ordering, as they did fine by me! PLEASE ADVISE: You will need to drill/grind the inner diameter to 3mm, as the production process often leaves behind bumps and material in the hole.