Recently I purchased a Northern 2500-D desk set dated 1971. The set uses a N.E. 35Q3AX polarity sensitive T/T dial pad. Since I have some simple, 2 wire AT&T polarity guard circuit boards, I decided to install one in order to avoid any future hassles in the event an outlet jack was wired incorrectly.
I wasn't able to locate instructions for installing the polarity guard on this specific telephone, so I asked "Unbeldi" for help in figuring out the circuit connections. He very kindly provided me with precise instructions on how to connect the polarity guard circuit board to the telephone's circuitry.
I've taken an existing schematic for the Northern 2500-D, and added a small image of the polarity guard, and inserted it into the schematic, showing the connections required for this particular telephone.
The schematic is attached as both a "jpeg" image, and a "pdf" file, for future reference.
Have a great evening.
Jeff Lamb