So I find myself in possession of five different tailcaps for Solarforce L2-type lights…I figured I might as well do something useful and write about them, with pics. 
Maybe someone will find it useful. Foy has already reviewed most of these, I believe…
The contenders (from left to right): stock late-model L2P tailcap, S5, S6, S8 and S10. The S5 isn’t common these days; I got mine from solarforceflashlight-sales.
Exploded view of the tailcaps in the same order, along with my $2 pair of belt-sanded needle-nosed pliers that make working with the retaining rings a breeze. I couldn’t get the white spacer out of the S10 tailcap, but it seems to be similar to the S6.
The late-model L2P tailcap uses a 14mm non-nub-type tailcap boot. The metal retaining ring was the same outside diameter and threads as the S5’s retaining ring, but the inside seal and inside diameter arrangement was unique in both the stock L2P and S5 tailcaps.
The S5 tailcap uses a 16mm, nub-type tailcap boot. I could not separate the white plastic seal from the metal retaining ring. The spacer between the switch and the boot was two very slightly dished washers with the concave side facing towards the switch. The two washers were not the same thickness; in my light the thicker washer was next to the switch while the thinner washer was next to the boot. The switch itself was unique and unlike the others…I’m guessing due to the tailcap’s age. The spring on the S5 tailcap was a bit more than 1/8” longer than the other tailcap switch springs.
The S6, S8 and S10 tailcaps share the retaining ring, the seal and the switch. The S6 and S10 tailcaps are both 14mm, non-nub-type tailcap boots while the S8 is a 14mm, nub-type tailcap boot. S6 and S10 use the same plastic spacers; S8’s is bigger. I’m guessing that’s why the S8 needs the nub-type boot.
I believe the stock L2P tailcap shares the plastic spacer with the S6 and S10 tailcap as well.
Why do I have so many tailcaps? I had a hard time pressing the button on the L2P tailcap with my thumb, so I originally got the S8 in hopes that it would be easier…And it IS easier to reach the button, so I’m satisfied there. The others showed up cause I wanted to see how they differed from the S8 and if they’d work any better without being as bulky as the S8.
The S5 seems almost like an earlier version of the S8. It’s a slight bit taller than the S8, but not noticeable unless you put them side by side. I actually like how the S5 radiuses in to and fits on the body better than the S8…If you want a more “smooth” light with less grooves in it, the S5 might be a good option…But get them while they’re still around.
The stock L2P, S6 and S10 caps are very similar; not much different except for the area around the boot and the grooves in the S10, which aren’t present in the L2P and S6…
I’m sticking with the S8 on my EDC light. The belt clip and o-ring below the belt clip fit conveniently into the groove left by the S8 and the body, the button is easy to hit and the fatter tailcap keeps the light from slipping out of my hand.
The S10 tailcap on the “blank” L2P body tube (which looks like an error or factory second, cause there’s some blemishes in the anodizing) with the L2T head is going on my AR-15. The S6 and S5 may go on to some lights I’m giving as gifts for the holidays…That’s really up to the recipients. I’m ending up with an increasingly large stack of left-over late-model L2P tailcaps.
If anyone’s interested in any particular measurements of any particular tailcaps or pics of specific tailcaps side by side, I’d be happy to do that.