I adjusted the tachometer using the optical tach from the association. It was super-easy and showed that the tach was 450 rpms off. When I took the back off the instrument panel, a nut and washer fell out. As far as I can tell, it was on a bolt above the two bolts securing the instrument panel to the binnacle. There's no bolt sticking out anymore, but I think I see the shadow of a bolt inside this third hole. I took the compass off, but the inside of the binnacle is a maze that I don't want to get into. Does anyone know what this bolt is for?