I bought it and the way it attaches is it screws on to the head like the bezel ring. On the L2 you remove the bezel and screw the lantern on to the head and then you screw the bezel on top of the lantern. I got mine from ebay in a package deal that included the lantern and a whole l2 head for 16 dollars shipped. Because i got the extra head and i already had an extra tail cap, i went ahead and bought a new stye 18659 body so now I'll have two l2's one that can take 18650s.
The truth is that ceiling bouncing your flashlight onto a white ceiling will light up a room better. This lantern has a white diffuser and then a cone reflector which probably causes some light loss. However, if you are using it outside where there is no ceiling or if you have dark ceilings, the lantern might be usefull for you.
I understand that the surefire 6p head and the l2 head are compatible so i think it should work on the 6p though i don't own one to confirm it.