I would say that it may be a crap-shoot as to whether any 18650 body already in your possession fits, there were some incompatibilities early on. I would hope that they have nailed things down and are making things fully lego-able by now.
I’d think they’d do that to lower costs. Identical threads means identical setup and inspection for that portion of the light, which should result in greater profit.
I wonder how the UI works? From the illustration it looks like you have to click through all the modes before the light turns off. I hope this is not the case.
what would be a good choice of 18350 cell for this, xtar 18350 850mah unprotected, BASEN 18350 800mAh 12A. Trustfire TF 18350 Li-Ion 1200mAh Button Top 3.7v Unprotected, EFEST 18350 Batteries 3.7V Li-ion 800mAh IMR, my guess unprotected would be best.
Note they are somewhat ‘old’ - four/five years. There has not been much evolution in the 18350 format but more recent cells may have a bit more capacity (those test at 700 mAh). Make sure you buy new ones because LiIon cells do age even without use - or so is my understanding.