Good evening all .... Here is a schematic diagram in "PDF" format for the Northern Electric 500-C&D sets, for individual line, party line and Key system service. February 1970.
Also attached, are the instructions for the identification, installation and maintenance of Northern NE-500, NE-1500 (MD) and NE-2500 non-button and one button type sets, including suffix identifiers, tables and trouble shooting information. November 1973.
Enjoy the rest of the weekend.