I have written to Great Planes about what I believe is a manufacturing flaw in their medium size nylon control horns, but get no response from them other than to mail the parts to them. I believe the backing plate has a mirror image molding such that in order for the plate to be aligned properly with the main control horn holes, the plate would have to be installed upside-down. That places the reinforcement bumps down into the control surface, which deforms the surface. The pictures to the left should make clear what I am talking about.
The control horn on the left in both the upper and lower images has the backing plate installed with the reinforcement bumps facing away from the control surface - the proper method. However, this forces the backing plate to be skewed in alignment relative to the main control horn part. The control horn to the right in the upper and lower images show that the only way to obtain proper alignment is to install the backing plate with the reinforcement bumps facing into the control surface. Doing so crushes the surface, and if the balsa is very hard or plywood is in the control horn mounting area, the backing plate ends up sitting on the bumps. This is clearly not the way it should be.
Other size GP control horns do not exhibit this problem. It is totally unbelievable that I first contacted them about three years ago and they still ship them like this. If you have seen the same thing and believe the parts are molded improperly, please send me an e-mail at the above address. I will collect the opinions and send them to Great Planes.