Your man is right - the hinges are all the same (apart from left/right), but they very easilly get bent by people not keeping them oiled. Once distorted they are almost impossible to straighten because of the very strong spring. It is best to pick good ones yourself from a car where you can see that they are undistorted. There is a certain amount of adjustment possible at the mounting points, but if the corner sticks up by half an inch I have never really managed to get one flat again.