The source material for tonight’s trivial question for Sunday Evening 6m Net comes from a 1994 QST article Overview of Amateur Call Signs, An — Past and Present. The article is a fascinating look at the history of how amateur call signs were assigned in the US up through the present regions and assignment schemes (except for vanity call signs which started in 1996).
Note: The article requires ARRL membership and a login to their website to view the archive. If you login to the ARRL website first from the main page and then click the link above, you should be prompted to view or download PDF of the article. If you have back issues of QST, it’s in the May 1994 QST on page 54.