It looks like it's 90% undone.
The plastic should be a separate part which houses the driver circuit. There is probably a metal heatsink for the LED behind the plastic and some wires between the two materials.
My initial thought would be that the plastic base should just pop off since threading would twist the wires between the board and the LED. However, I would start by looking closely between the plastic base and metal head (seems like there is a decent gap there) to see if there is anything resembling starting/ending threads, a snap/spring ring (metal ring with a gap), or maybe pins and notches (like a bayonet type light socket).
Since the threaded metal part you removed from the bottom of the head looks like it retains the plastic base part, a second securing method would be redundant and overkill for a non-strained part that has no thermal function. Since the plastic is not something that can be replaced perhaps someone with a successful disassembly of this model will chime in with a proper method to separate those parts.
Another observation I just had looking at the photos is that the plastic base has 4 angled notches near the battery contact. It's possible Surefire uses a special tool that fits in those notches to help grip the heatsink and plastic driver assembly to thread them into the head.
