Actually that rectangle hole was one of my designs on the "exact" reproduction GNX dashes sold some years ago. It used a special round pop-in steel plug that mounted in the unused bulb hole, with a spot-welded and threaded L bracket, with a hex nut to hold the pickup opto head. The cases had to be opened, and the rectangle had to be cut into the back, then re-crimped. Was a real PITA.
Those rectangle holes weren't on the factory speedometers.