1966-1967 Pontiac GTO/Lemans Tachometer
I was asked to fix one of these tachometers, which led to me reverse engineering it. Here are the schematics for it:
Schematics
The box in the middle is a mystery 4-wire semiconductor device. Evidence shows that this is most likely a plastic hybrid component with 3 diodes in it. This appears to be a Dixco design, but it is not branded. You can read the reverse engineering report on this tachometer here.