Wasn't the 56/64k thing a result of CAS (bit robbed) signalling which was a fudge AT&T did to transport signalling information in-band on T1s by stealing the low order bit for OOB signalling (it wasnt actually every low order bit, but meant you had to throw away every low order bit as CPE didn't know which ones were "corrupted" by the carrier).Proper ISDN was always 64kbit/s clear path with separate D channels carried OOB end to end, away from the B channel data.
Soon after commercial success of T1 in 1962, the T1 engineering team realized the mistake of having only one bit to serve the increasing demand for housekeeping functions. They petitioned AT&T management to change to 8-bit framing. This was flatly turned down because it would make installed systems obsolete.