I suspect the same formula that lets you use a smaller ground in 250.122 for sizes over 10ga cable (ungrounded conductors) was extended down to the "small conductors" in the 50s. My 75 book has a table that looks like 250.122 except it is 250-95.
Now if I go back to the 1959 code, I find the smoking gun. I looked to see exactly when it changed. 1965 still has the 16ga wire but the 68 code has it as full circuit conductor size for 14-10 so it must have been in the 68 ROP.