Thanks, tsuwm...Quinion, indeed, has the whole nine yards on the whole nine yards! Just like the full monty, numerous theories but nothing certifiable, no cigar. He does say the WWII ammo story may have "merit" because its the right time-frame. And with even more pertinence to this number thread, he had this to offer:

Some writers argue that the number isn't a dimension of any kind: Jonathon
Green, in his Cassell Dictionary of Slang, suggests that it's most likely to
represent a use of nine as a mystic number, after the fashion of nine
tailors, the nine muses, and several other expressions; Jesse Sheidlower
thinks that it may be related in this way to the number in the equally odd
expression dressed to the nines.

Here's the link: http://www.quinion.com/words/articles/nineyards.htm