I'm trying to change the pickguard on my Lakland Dunn Skyline (manufactured 2007) and having a devil of a time getting the knobs off. Are they held onto the posts with a set screw? There is a screw hole, but I cannot (1) see any screw head, or (2) get any purchase on a screw using tools of different sizes and blade shapes. At the same time, there is very little give if I pull hard on the knobs. I don't want to force anything. Does anyone know for sure whether the knobs have set screws? If so, what size and type?