Hi Bill
If I may venture a guess....
USAF was created in September 1947, so either the "bosses" had already worked out that they should refine the old USAAF designation system and wanted it to be started in fiscal year 1948 (which I think started on July 1st 1947?), OR they were late and made the decree for fiscal year 1949, which then should begin on July 1st 1948. If the manual quoted earlier is anything to go by, the July 1st 1948 date seems the better one.
It would make sense if you did start on a date like this, but of course nothing with dates like this is 100% certain unless you locate the specific order. Nothing in my books lists any specific date for this refinement, so again, purely a guess
Cheers
Stig