Mirror Image

Mirror images have the exact same AC and and AC type as the caster.1) This was changed in later editions of the game, but in AD&D, the images are “exact duplicates” (PHB 71). And since they are an illusion spell, they have the full power of illusion. That means if the attacker's weapon “hits the armor” but doesn't score an actual hit (due to the caster's AC from armor, or due to WvAC), the illusion causes the attacker to believe they might have struck the real caster's armor but did not strike a telling blow, and the image remains.

Targeting

For targeting purposes, since the mirror images are exactly mimicking the caster, they do not count as separate targets for target determination. The caster remains but one possible target, and if the caster is struck, then a die is rolled to determine if the caster or one of the caster's images was actually struck.