Posted 6 years ago
BSPs (Bell System Practices) are useful to phone collectors. They contain technical details on the model numbers, features, connection and servicing of Bell System products.
They were published from about the 1920s until about the turn of the century in several loose leaf sizes, suitable for updating as features changed, and in bound form -- primarily in compact sizes for installers and other field personnel.
More details here:
Please contact me if you have some spares or readable scans to share.