Steve,
I believe that the problem here is that the "servo booster" devices are not common on this side of the pond. The booster that we normally see mounts behind the master, boosting the force into the piston, while the "servo booster" takes a hydraulic input, from the master, and using a similar vacuum diaphragm boosts the output hydraulic pressure to the wheel cylinders. They look to be a pretty slick unit, and here's a link that I found that gives an overview of the operation - Remote Mounted Brake Boosters: Vacuum Booster Solutions For Cramped Engine Compartments | Master Power Brakes