I have been using the Olight ORB-163P06 (RCR123A ) protected cell in my M3, if that is any help, My cell measures about 34.5mm long.
This is the Amazon listing Olight cell
I don’t possess the M3-pro yet but chances are pretty good this cell will fit the M3-Pro as well.
I am not completely sure of what I’m saying, but to get the full power out of the M3, unprotected cells are required. That cell from Olight is protected.
Here you can read that it doesn’t work well on the Olight S1R Baton (900lumens) due to protection kicking in. [Review] Olight M1T Raider (500 Lumen, Tactical EDC CR123A) - #12 by rookiedaddy
The M3-Pro has, supposedly, more lumens, so I’m afraid that won’t give you enough power to feed that light on max!
Cool pic. You can tell the reflector takes a lot of space, then I guess there’s no recharging circuit near the driver which also saves space. They really crammed it tight. Lol