@cannga - I tested my S21D (w/ 219b emitters) with the IF25A optic. For me, this is a major improvement. It gives a truly defined hotspot and makes it much better for general purpose use. I appreciate you doing the research on this and sharing your findings, as I’m very pleased with the results.
Strangely enough, I think I prefer my IF25A with the optic that came with the S21D, so it’s a win/win. Are you aware of any other optics that fit these two lights?
On a different note, have you noticed any strangeness on your S21D in terms of brightness? I’ve tested two groups (#1 and #5), and the 100% on both is NOT max brightness. If I cycle through the levels, every now and then it will jump to what I would refer to as “turbo”. The easy way to tell, aside from the increased brightness, is that at 100% the light head does not heat up to uncomfortable temperatures even after 10 minutes of running. At the “hidden” turbo setting it gets rather hot after <1 minute. The light does not remember this setting after turning it off - it reverts to whichever setting it was on before you slipped into “turbo” and you have to fiddle with it to get it back there.
I would describe the perceived brightness difference between “100%” and “turbo” as roughly 2x. It’s very noticeable. It seems to be easiest to get into this mode by holding the button in just before the amount of pressure normally required to cycle modes, then releasing once it switches to “turbo”. I have run it at this setting to the point of it getting as hot as my single LED Convoys get (not too hot to touch, but getting uncomfortable) and it has not released the magic smoke.