The two letters were not mnemonic, but were the Bell Telephone “name” for the analog switching machine building. Thus, our old San Antonio TX (and two letter state abbreviations are also a “new thing” !) phone was Oldfield 5-0577. 655-0577).
The Bell Oldfield exchange building had floors and floors of switching machines. OL1. OL2. OL3, OL4, etc.
A smaller town (one earlier writer used Davenport IA as an example) might only have one building and two or three switching machines.
So, “tracing a call” literally meant hand-looking up every mechanical switch position for the seven numbers in the exchange, and writing down each contact position one by one. If the criminal or spy hung up before all 7 switches could found, the rotary contacts reset back to 0 for the next phone call.
Good comment, thanks. One of my Texas uncles worked for Ma Bell, had one of those old switch boards. Looked intimidating.