I called Happ technical support and asked if there was a different way of mounting the Competition in a metal CP than in a wooden CP and he said "Yes, you use the shorter nylon spacer and the upper E-ring groove." I told him that mine only came with one spacer and it was a long one. So he checked and sure enough, they only come with one spacer. So then he thought maybe you were supposed to leave the spacer off altogether. I told him that doesn't work either, because the actuator slides right past the 2nd E-ring groove. He looked through his materials on the matter, couldn't find anything useful, other than it saying "for wood and metal control panels"; at which point he said, "I guess they are just expecting you to live with the extra height of the joystick when mounted in a metal control panel."
He also didn't know what the 3rd (uppermost) E-ring groove was for either.
A shorter nylon spacer would work if there was one. Maybe they ditched it to save money. He suggested cutting the existing long spacer down but I'd rather not do that.