Got pics?
Cloud makes a mini/compact version of their REIN which takes 18350s. It WILL NOT work with a CR123. You can get inexpensive 18350 cells
here.
Just going by the markings you listed, though, at least that head came off of a MCH, or "Mission Configurable Handheld". You can still use it on a REIN body, but it's not a REIN head. It
is still fully potted, but I wouldn't use it as a weaponlight due to the fact that it can be configured to have different outputs, unlike the "on/off" output most people desire in a dedicated weapon light.