Looking at my oldest code book (1978) 384-3(f) "...The B phase shall be that phase having a higher voltage to ground."
There's no 384-3(f) in the 1971 NEC, nor can I see any other applicable reference within article 384.
200-6(c) in 1971 covered general identification of the high-leg:
Where, on a 4-wire delta-connected secondary, the midpoint of one phase is grounded to supply lighting and similar loads, that phase conductor having the higher voltage to ground shall be orange in color or be indicated by tagging or other effective means at any point where a connection is to be made if the neutral conductor is present.
The part I've quoted bold has the "changed from previous edition" line alongside. I seem to recall a discussion a while back in which it was decided that 1971 was the first time orange was suggested specifically.
Still nothing about the high leg having to be B phase that I can find though.