In reading a book about the Key System railway, older schedules had a
> six-digit phone number (2L-4N) while newer ones had seven-digits > (2L-5N).
> Would anyone know when Oakland converted?
My guess is Oakland was converted to 2L-5N prior to the introduction of DDD (direct distance dialing.) That's what happened with Seattle which was 2L-4N prior to being converted to 2L-5N. Also, often when converting from 2L-4N to 2L-5N the central office "exchange" code was changed. The change in Oakland was likely done in the early to mid-sixties since that's when DDD was introduced to most of the big city areas.
Also, was six digit dialing (2L-4N) common in a lot of places?
2L-4N was very common except in exceptionally large cities such as Los Angeles, New York and Philadelphia though I bet that originally these cities had 2L-4N as well.
[TELECOM Digest Editor's Note: Chicago was 3L-4N throughout the 1930's and 1940's, (that is, from the start of automated calling through the final cutover of same.) The original third letter (as in ALBany and ROGers Park) became the first number in the new 2L-5N system, (which is to say ALBany became ALbany-2, KEDzie became KEdzie-3 and ROGers Park became ROgers Park-4. That cutover occurred, I think, in 1948 when the final conversion from manual to dial took place in the HUMboldt central office. Operators were spreading rumors that after the final cutover, 'hundreds' of operators would be laid off, out of a job. The fact is _no one_ was laid off, and with the opening of Ohare Airport six or eight months after that, Humboldt had more employees assigned there than it ever had in the manual days. PAT]