Incidentally, the threads in the old RRT-01 are compatible with the threads in the Manker E14 / Astrolux S41.
This means you can take an 18650 tube from those lights and lego it onto your old version RRT-01 head to get instant 18650 support.
There also tailcaps available for that thread pitch without without switches on them. So if you want a flat back with no switch it’s possible to do. I just tried it with parts on hand and it worked.
While the threads on those lights are the same as the threads on the RRT-01 head, it should be noted that there are more threads on the Astrolux/Manker tubes. If you want it to be watertight and cover the o-ring on the body tube you may need to shorten the threads slightly by sawing or filing off the top of the battery tube.
legs wont fit into the Original RRT-01
trimmed leg sides on non-carclo triple optic for RRT-01
the non-carclo brand narrow clear optic sold by kaidomain is modified to fit without any glass lens over the optic, because the optic has a smooth face
when using a carclo 10507 besides trimming the sides of the legs as pictured, it is necessary to install a glass lens over the optic, to cover the holes on the non smooth face of the carclo narrow clear optic.