The Bell System, being, essentially, the only game in town, and having their own vertically integrated company, did not need to print or distribute details that could fall into the public's hands. There were no component-level field repairs expected, therefore no field repair documents.
Some engineering documents can be found, in excruciating detail, in the BSTJ, and the BTR. (Bell System Technical Journal and Bell Telephone Record.)
After that, there are SD's (Schematic Drawings...you displayed one above) CD's (Circuit Descriptions) and of course the lowest form of documentation, meant for the repair force's field reference, the BSP's.
As time went by and the repairman's job was dumbed down, the schematics in BSP's were also simplified, as you have seen, when looking at what you know to be a complicated circuit, but shows just a few block diagrams where "a miracle occurs" as the old cartoon stated.
Those of us who worked in the golden age of 1A2 and EM PBX's jealously saved and hoarded the original, detailed, BSP's and would refer to them when the "new & improved" versions didn't offer the detailed help that was needed.
[img]
https://i.imgur.com/EXmmL.jpg?fb[/img]
