We were testing compatibility between some of the components of a real Surefire M961 and a Element M951 current-gen (both lights are a simple matter of unscrewing the head and tail cap from the body), out of curiosity. The following are the results:
Tail cap fits and functions when done swap between both Surefire and Element parts. Of note is that the Surefire push button goes momentary on at partial depress of switch, and can click on/off, whereas the Element switch is click on/of and when on can be partially depressed for momentary off.
Head isn’t quite a complete compatibility – Surefire body and Element head fits and functions fine. Surefire body will accept Element head, but Surefire head would not quite thread into Element body. The Element clone uses LED, so features a brighter and whiter output over the Surefire incandescent bulb (think cheap LED upgrade if already have the parts lying around), but offers no IR functionality when using an appropriate filter.
Remote switches do not appear to be compatible.
Light belongs to “Jetlag” – I just took photos of the trials he was doing with the repro Element light from work.