relay powered voltage regulator ?

That is good information. It is always good to know the relay by reading the data sheet, or imprints on the relay. Wire per relay and task. Since the diode is in parallel with relay coil, it might be difficult to use meter to locate polarity of diode.

I buy relays from places like Digikey, Mouser, or Partsexpress. I avoid the high priced junk, often found at auto parts stores wiring section.