This light has been long since discontinued, but the W117C model was designed to be mounted to 1911s WITHOUT rails.
You have to buy the surefire rail seperate, and attach it to your 1911.
My question is, can this model be modified/adapted to be used on 1911s already with a dawson precision rail installed?
Now all light rails from Dawson specific to the SureFire X200 only , but the W117C/D can attach to your 1911 if you are using the early Dawson precision W117D light rail . 
The specs on the rail itself are the same for the w117 and the x200/300, and once installed on the pistol you can switch between the x200/300 and w117 lights with no issue. Dawson no longer lists the w117 as an option because the lights haven’t been available in a few years and very few of them are out there. Do you have a w117 with the surefire interface for their bolt on rail setup? If you have the light with the proper interface call them and they can hook you up. If you need pics of anything let me know.
Dawson no longer manufactures the Dawson Rail adapter for the Military Light, but if you can find it…all it requires is removing one pin/screw.