I've been comparing some code sections between the current NEC and an old edition that I have.
From the 1971 NEC:
210-5. Color code for branch circuits.
(c) Ungrounded conductor. Where installed in raceway, as open work or as concealed knob-and-tube work, the ungrounded conductor shall be identified by any color other than as specified in (a) and (b) above. All ungrounded conductors of the same color shall be connected to the same ungrounded feeder conductor and the conductors for systems of different voltages shall be of different colors.
There's a bar to indicate something here was new in 1971.
1. 210-5(c) no longer exists in the 2002 NEC, although it appears that something similar has been added to 210-4 where it applies only to multiwire circuits. So when was 210-5(c) deleted?
2. I'm not sure if I'm reading this old rule the right way or not. The second sentence seems to prevent the use of Romex on a 2-wire branch circuit fed from a normal 120/240V 3-wire panel, unless all the black wires on one side of the bus were reidentified to some other color.
Or was that second sentence only ever meant to apply raceways, open work, and K&T? If so, then it doesn't seem to make that very clear from the wording.