A diesel motors was an option in the same van models which came with a
tachometer if they were powered by a gas motor.  Did the factory provide a
tach in those same vans if it was powered by a diesel?

I've seen after-market diesel tachometers which use a Hall effect (moving
magnet sensor) transistor to key them as an indication of a shaft's rotation
if a magnet is attached to it.  But a tach that matches the Vanagon dash would
please me more.

John