Quick observation: S21E is compatible with tailcaps from Wurkkos and some Sofirn 21700 lights, while M21 series is definitely not. I've checked it with TS22 and TS26S tailcaps, they have good magnet and wider spring too. I assume that all modern Wurkkos 21700 tailcaps will also be compatible, as well as tactical tailcap from Sofirn SP35T.
by DarkSideOfTheCree
