On the picture on the right side of the top, there is a outer ring, and from top down, you'd see an inner ring. This let's you use 510 atty's and if you remove the outer and inner ring, you can then use P3 threaded atty's. The only thing that is removable would be the two rings.
Perfect answer, I knew a little about the P3 connection but have no experience with it.
Thanks.