I like to make system-neutral stuff. I don’t particularly like writing “AC 8 (12)” or “AC 12 (8)” or “AC: as leather armor”. Here’s my proposal for a terse way to write any-edition Armor Class: Just write the AC bonus. Thus:
I think it’s pretty intuitive that this guy’s AC is 16 in 3e (counting up from 10), AC 4 in 1e (counting down from 10), and AC 3 in Basic (counting down from 9).
This system doesn’t collide with any other notation. “AC: -6” has meaning in other systems but “AC: +6” doesn’t.
It also works within the expectations of each edition.
Monster publishers: if you like it, start using it. If it needs a name, I suggest “Unified AC Notation” and you can link to this page.