Ah, I considered adding that one to my list, but I thought I'd be the *only* one to even know what they were.
At MIT in the 1970s', I helped build and maintain a switching system for our fraternity based on these. I think we had a rack of 7 trunks -- each with a linefinder, and one final 2-digit switch.
These were units being discarded after MIT had upgraded their phone system, so they were already at end-of-life. There were times I was glad we had 7 trunks; it made it a lot easier to keep at least a couple operational!