S1, S2, S10, S15 and S20 don't have separate heads and bodies, but their body is a single piece aluminum instead. However reflector, lens, bezel ring and pill seem to be the same size in S10, S15 and S20 and their rechargeable versions and should be interchangeable. I have S10 and S10R and their tailcaps do work interchangeably. I also found out that my new S10 has an S10R circuitry inside in the pill, so the recharging function works just by swapping the tailcap. That probably won't work for older S10 models though because the circuit board of R models have a diode that allows the current to flow in reverse direction through the pill and the battery to make recharging possible. The older non-R models may not have this diode and thus cannot recharge the battery and it might be a bad idea to try it because mechanical reverse polarity protection would then be bypassed and the pill would experience a reverse voltage. I found out that swapping that diode to a low voltage drop Schottky diode allows S20R charging dock to charge the battery to a higher voltage than before. First it stopped charging at 4.05 V and after the diode swap 4.23 V, which i think is actually too high for the battery. S10, S15 and S20 all have different diameter battery compartment as well as tailcaps, so you can't swap the tailcaps between those.
I have tested that S10R pill works in S20R body, but because it lacks the positive battery contact spring, the battery loses contact and the light shuts off if you shake it hard. This happens because 18650 is more massive than 16340. S20 pill should work in S10 or S15 too but the spring in the pill might need to be cut shorter in order to make enough room for the battery. The smaller battery might also have a hard time providing the 1.4 A current that S20 can pull. Although S1 pulls even more current, 1.8 A from a small 16340 cell. S1 and S2 have different type of bezel ring and different optics compared to S10 / S15 / S20 so those aren't easily interchangeable. I think S10 and S15 pills would be interchangeable without any modification, though i haven't tried it. S15 pill can operate with a lower voltage (about 0.9 V) than S10 pill (maybe 2.4 V) because it was made to work also with alkaline batteries.
Here's S20R disassembled:
The internal components in the head are the same size in S10 / S15 / S20. Only the body, tailcap, pocket clip and the spring in the circuit board are different in size (since only S20 has the spring).
Here's whats inside S1 head. It's quite different than what's in S10, S15 or S20:
http://www.candlepowerforums.com/vb...-L2-(R)CR123&p=4773794&viewfull=1#post4773794
