When I went to the NH State Library the second round to get some copies of the System 75 book, I found another AT&T Technical Journal with a tell all on the 5ESS switching system used for the phone companies. The book was twice as thick as the System 75 tell-all.
I found pictures of the system and copied them. I didn’t plan on taking several hundred pictures on my iPhone on the ESS. The system is ridden with two or three letter abbreviations, which one can get easily confused.