I can't find the photo in the Alternative Gallery, but from your discription, it sounds like a microlock switch. If this is what it is, it is an aftermarket valve that is installed in the brake line to set and hold the brakes on. Kind of like an emergancy brake. You hold down the brake and move the lever to set the brake, then to release it you step on the brake and move the lever to the off position. We used devices such as this th hold our wrecker in place when recovering a car.
Hope this helps
Jim