permeke, the "p" version of the output circuit was designed primarily to solve certain problems of the U.S. market in the 1950s and 60s, when many broadcast and recording consoles were still in use which had been designed for dynamic microphones, including ribbons. The output levels of professional condenser microphones were as much as 10 to 20 dB higher than the preamps in those consoles had been designed to handle.
In addition, the input transformers of some of those preamps were used in a circuit configuration which would cause audible errors in frequency response, and sometimes even parasitic oscillation, if the microphone's source (driving) impedance wasn't in the range of 150-200 Ohms. Thus the 50-Ohm setting of the microphone's output transformer might help with the input overload problem, while causing a frequency response problem at the same time.
In the "p" version of a Neumann microphone amplifier, the transformer is strapped for 50 Ohms and the bridges which you mention are opened so that the 47 Ohm resistors are in series with the transformer's secondary windings. This brings the microphone's output impedance up to around 150 Ohms and provides a further reduction in sensitivity, thus helping to avoid input overload while maintaining correct frequency response even with the problematic kinds of console preamps.
With modern professional preamps there is no need for the "p" variant of the amplifer any more, but during the reign of Gotham Audio Corp. as the North American distributors of Neumann microphones, the "p" configuration was the default--if you wanted 50- or 200-Ohm Neumann microphones you had to special-order them from Gotham (which I always did, and was always quizzed to make sure that I knew what I was doing).
If a resistive pad is needed to protect a preamp from input overload, that pad belongs at the input of the preamp rather than in the microphone. A pad at the input of a preamp attenuates any noise pickup as much as it attenuates useful signals. A pad in (or at the output of) a microphone reduces its useful signals while leaving any interfering noise unaffected. When you turn up the preamp gain to compensate for the pad, you raise the noise level even further. So the "p" circuit arrangement is really to be avoided nowadays.
--best regards