OK, you called me on it ...
On the FAA's website they have
effectivity and amended dates for each section of each part.
For 91.159 they indicate an effectivity date of 09/30/1963 ... the original "delivery" date of Part 91 (pardon my high-powered legalese). However they add the comment, "
The historical amendments of Part 91 are not included at this time. Part 91 was entered as of Amendment 91-255." I take it to mean that this section, pertaining to VFR cruising altitudes,
could have been amended to mandate the east-west convention sometime prior to 91-255 (issued Dec 1997), and, maybe ... possibly ... sometime after 1963.
But you'll have to tell me what was in use before the current convention to convince me of that.
(And I'll never apologize for being "that guy")