There was a factory option to put a hydraulic remote on a 550, and it would have the 2 hose, double action like you mention. They can be hard to find these days, but an external aftermarket valve can be added, which is what I am guessing someone had done in the past from what you describe. The external valve has to have its own relief valve built in or you will blow up the hydraulic pump. It is an open center hydraulic system.