The LED Zep LED mods for the SF M6 are not drop-ins in the same sense of a P60 drop-in for the SF 6P family. The LED Zep product is mot an LED module that you drop into the host body and then screw on a bezel. The LED Zep product is a mod of the KT4 TurboHead itself, converting it into a self-contained LED head -- LEDs, reflectors, heat sink, driver all encased in the KT4 shell. Then you simply screw the mod'ed TH onto your M6.
The cost can be high, especially if you opt to mod an extra KT4, such that you still have the original KT4 if you want to go back to stock operation. You'd have to buy the extra KT4, which can be around $120-$150, on top of the cost of the LED Zep kit. Also, the driver is extra cost as well (IIRC, LED Zep can supply one, or you can source one yourself). In any case, IMO, you get what you pay for. The LED Zep kits are of the highest quality with matching performance.
I got an MZXR-7 kit from LED Zep. It included the heat sink, universal contact, 7 XR-E R2 LEDs, and 7 reflectors for the XR-Es. I supplied the thermal adhesive, Teflon hookup wire, KT4, and Blue Shark driver. LED Zep also can open the KT4 TH for you, saving you that hassle.
There are various Millennium-compatible LED heads. The Lumens Factory product has been mentioned. OpticsHQ has their TX3 and TX4 heads.
Also, as mentioned before, nailbender has a "drop-in" LED tower that fits the SF KT1/KT2 and KT4 TurboHeads. AW has an LED tower kit, available on the CPF Marketplace. Netkidz may still have some of his kits available (or maybe nailbender bought them all).
