AT&T's publication "Notes on Distance Dialing" (1975) [1] includes a
> list of 63 NN0 codes that could be assigned either as area codes or as
> central office codes. This list, identified as "Chart 5," includes all
Hmmmmm ... I distinctly remember 212-440 (Manhattan), in the first part of that list and 212-680 (Brooklyn, then in AC 212) which is toward the end of the list as both being in service in the early
1970s. If that list was around in those days they didn't appear to be following the order. ;-)