What is compelling about what the OP calls "solid" tailcaps is not that it is a twisty, though it is, but that in the right shorter cap makes the length of the flashlight considerably shorter, and what makes it more compelling, if it is solid and springless, is the lower resistance. As such, this style tailcap may be referred to as a zerorez or even a "
shorty," (not to be confused with the eponymous nickname for a tail-less short body replacement, which is really the same thing, the tailcap becomes the body) to go along with the tailcap
y nicknaming motif, ala twisty, clicky, mounty, flicky, rotary, dimmy, ringy, sticky, itchy, scratchy,
et al.* One might say that all ZebraLight tailcaps are shorties, though they have springs and more resistance than the solid variety. I think there is some good info
here, and better info
over here.
I'm just making this up, so YMMV
*glowy, shroudy, gritty, grippy, knurly, slippy, icky, minty, tricky, nifty, McClicky, heavy, jaggy, snaggy, springy,
tritty, shiny, spinney, snappy,
litty, blinky, flashy, floaty, tachy, remotē, stealthy, grizzly, beefy, jelly,
twofitty, fuzzy, sweaty, rusty, toasty, puffy, iffy, dinky, flimsy, squishy, wiggly, faulty, lengthy, boxy, really?!, ORLY,
etc.