Robert - you are welcome.
The picture shows the tailcaps are interchangable in terms of fit but there are several issues.
The Pelican tailcap will work on the Surefire (C2 in this case) with full function (behaves like a clicky.) There is that gap between the tailcap and body but the o-ring is covered though, but I would not trust weather resistant ability. I tried this tailcap on a G2 and it would not work (something about the G2 having nitrolon threads while the C3 has metal ones.)
The Surefire tailcap fits and only works "constant on" on the Pelican light. There is no momentary. I also had to tighten it all the way down to make it work, then sometimes it would not turn on. I think the tailcap spring is too short while the Pelican tailcap has a metal piston type thing and not a spring.
The bezels and lamps will not change due to thread and size issues. (Pelican has a wider head that tapers to a msaller bezel/body contact, while the Surefire remains the same. The Pelican lamp will not fit in the Surefire bezel due to being too big, while the SF lamp fits in the M1 head but has space to rattle around (talking very small amounts but noticable.)
This all being said, I would not place the Pelican tailcap on my Surefire and leave it on because of explosion issues. The P91 lamp draws a good amount of amps and I don't want my cells to vent on me. I am not saying it will happen but I do not want to increase my chances.
Jason