@LostInParadise I wrote sloppily in my first reply here. It’s always been the rule that you use “a” with H when it it pronounced: a hose, a hat, a habit
According to this article it’s actually about the emphasis being on the second syllable, which apparently used to result in an H not being pronounced (...), and the “an convention” lasting longer than the previously-silent H on those words.
So I (hehe) would say my instinct is correct that saying “an historic” would make more sense as “an ‘istoric” ... in my mind it goes along with the accent that would also say “an ‘orse” (conjuring Pygmalion & Audrey Hepburn in My Fair Lady).