I have 3 SF scout lights currently.
-Old KL4 M600
-Fairly new KE1A M300
-Fairly new M600 Fury

I have an old pressure switch tailcap from the KL4 light that won't screw on to the newer M600. The threads start and then hit something solid and it stops.

Looking at the newer M600 it appears to have some kind of different metal (steel?) insert pressed in the back as pictured below. The other two lights do not have this. Does anyone know what this insert is, and why it's there?

M300 without insert


M600 with insert

A little while ago, Surefire changed the thread pitch on the scout lights. There were a couple old threads about it.

I did some googling and found similar information. As far as I can tell it's not the thread pitch as the 3 other clicky caps I have are compatible between all 3 lights. It seems that the metal insert reduces the ID slightly and interferes with part of the old tape switch tailcap, but not the other caps.

Just looked at a friend's DF and confirmed it has the same insert and the old tape switch cap will not fit.