This is a tritium vial holder to be used as a pendant.
The holder is sized for a single 22,5mm x 3mm tritium vial.
To fix the vial in place use a small amount of borland 61 or similar optical adhesive.
This includes only the holder and nothing else, so you have to get the vial and an adhesive of choice separately.
Due too the printing process you may have to clear the vial channel of burs or other imperfections to make the vial fit.