GU Gearbox Door Lock Centre Case Old Style Replacement uPVC Mechanism 35mmGU Old Style Gearbox 35mm A replacement centre case gearbox for the GU old style multipoint door lock mechanism. If you're having problems with your door lock, the gearbox is almost always the problem and changing it to a GU Old Style Gearbox is as simple as removing a few screws. GU Old Style Gearbox Information Important Measurements: 35mm Backset 92pz Centres This is the mechanism inside the door that is operated by the handle. Most likely to be