Oil Pressure Sender electrics?

Actually, that may indeed be close! The gauge system really comes from the older 6 volt systems and that may be what your '51 had. That is the reason for the regulator... to drop the voltage to the gauge/sensor system to simulate a 6 volt system.

Here is a link to very similar VW thermal gauges to see how these work; the regulator described works identically to the Mopar regulator.
Electric Fuel Gauges