If you look at the Surefire head area, the bottom inside has a taper that gets narrow as it gets to where the battery is. The Solarforce body is a straight cut. The Surefire lamp assembly uses a large spring on the outside which will make contact to the Surefire body in this tapered area. Replacement assemblies from other manufacturers have a larger diameter spring. If you put a replacement LED module in the Surefire body, the head assembly will not screw all the way down unless you remove the larger outside spring. Conversely, using a standard Surefire lamp assemble in the Solarforce body requires that the large spring be bent out a little, really just a little bit, to make contact. I made an aluminum washer and sent that along with any bored out bodies.
I spent some time with this problem as the dimensions on the two bodies 'look' to be the same. My calipers do not go that far down into the tapered area to measure, measuring at the top the areas are the same.