This is the ‘official’ wiring information from the Reslo RB brochure…
As you can see, different models had different wiring conventions and required different colour coded cables. This can cause confusion, particularly if the mic has been separated from its original cable in the 40 or so years since manufacture. The most important thing is to check the mic and cable wiring and make sure that they match!
Unfortunately, grounding the mic by connecting the cable screen to the body of the plug does not work well, and these mics are prone to hum. For the low impedance models, it seems sensible to use pins A and B for the balanced output, and pin C for ground. The ground must also be connected to somewhere to the body of the mic and/or to the shell of the cable.
The dual impedance models are particularly problematic, in that pins B and C were used to select either low or high impedance output. This means that if one pin is used as a ground then one part of the transformer will be grounded and some of the signal lost. It is therefore important to disconnect the high impedance leg inside the mic and rewire to match the cable.