I just looked at both of my M4 Devastators. They didn't come thru with the tail cap that yours did. Both of my lights came with the XM00 and XM10 dual tail caps.
The XM00 is without the tape switch and the XM10 is with the 10" tape switch.
I believe that without the cable, the light won't turn on even if you screw the cap all the way down.