I have been searching for a while, and I cannot find any information on my rear u-brake. It has served me well for many years (10+), and has a surprising amount of force. It's stronger than any cantilever and most V-brakes I've ever used or felt. Adjusted for maximum power, it has crushed a few single walled rims for me, so I usually adjust it off peak for the sake of my rims (though it would probably be fine with dual walled ones). This isn't to say it's phenomenal, it's difficult to get wide tires through it, and if it's not adjusted properly it will hit the knobbies on large tires.
I have searched every millimeter of it for any markings or lettering, and it is devoid of any. If anyone has seen one like this, or can identify the brand or model, I'd like to know more about the company, the design, and the specifications.
With Brake Booster Removed
The closest I've seen yet is a roller cam u-brake, like the one below, only without the rollers:
Roller Cam U-Brake
They weren't original to the bicycle, but the frame is a 1997 GT Bravado, the brakes were purchased around 1997-1999. It came as a 1 1/8" threaded / 21 speed setup.