It's an old Surefire P116 6v light. Surefire discontinued them years and years ago. They originally came with a mount that clamped around the trigger guard (MR06 I believe again no longer made).
My set up is lightly modified but allows removal and instillation of the light in seconds. I used a TR rail (known as the Tick). The TR rail fits into the gun using the take down button and the front of the trigger guard as leverage points. This means with the push of the take down lever button and pulling the rail down you can easily pull the rail off.
The TR rail almost fit the P116. Some slight modification was needed in two places. At the rear of the mount where the back of the light indexes with the mount there is a slight lip that I needed to flatten down. I used a dremel for this. The other modification was to the number two rail on the mount. I had to shorten its cross section by half. Essentially making it a skinny rail. This again was done with a dremel. I made the cut then put an edge on to it to ensure the rail would lock solidly with the bracket on the light. I cut as little as possible so getting the mount locked onto the light was tough and as such the light and mount are semi permanently mounted together now. This entire process took maybe 5 mins.
I am sending the light off tomorrow to have an LED upgrade fitted to it. The result should be a 300lum LED light. It will look like an old retro light but work like a modern X300 light.
These lights are hard to find. Every now and then they pop up on eBay. They typically run about $100. The TR rail is $50 and the LED upgrade is about $50 so it isn't less exspensive than a standard X300U but to me looks a lot cooler then the MR11, DG16, Surefire X300 combination that I was using before.