What brand body, tailcap and lens housing?

Great drop-in.  I use one in a custom made E1 host made to fit a P6 tail switch using a VME head.
Brass head is a little heavy for pocket carry and I will eventually get a malkoff 3 mode head made for the stock E1 since they are smaller and aluminum.

As to your question.  You can still find P6 hosts bored out to 18650 if you look around.
Otherwise fit it to a Solarforce host (aluminum foil or outer spring) with one of their forward clickie switches.  I like the L2m though would like to try one of their LTM or whatever that looks like a Z2 for a handheld light but it seems redundant since I have Z2 with Lumensfactory warm XPG.
For an even better switch you can fit McClickie switch in Solarforce tail.  A little modification may be required.  Instructions are on Candlepower forums or you can buy one already installed.  Good for 5 amps.  Original Solarforce switches are a little less amp but are at least a couple amps over the draw of M61.

Forgot PLS-1 mount.  You can slide it off rail and use as flashlight too.  Spring loaded lever releases it from slots so it slides along rail to end and off.

Surefire 6P (around $55 on ebay) and a Surefire M79 mount (around $25 on ebay)

This.