The original idea seems to have been that defenders also make an armor throw against the attackers attack score. Since you have to add AC to the die roll and the attackers attack value is the target number.
Sorry, what?
I think. I still don't understand how you made attack rolls for the first 25 years.
Pretty much the same way we do today. Just expressed differently.
In OD&D and AD&D 1E it was with a table lookup, but you're still rolling a d20 and trying to get high enough to hit the AC, though lower ACs were better. There was no auto-miss on a 1 rule or auto-hit on a 20 (I believe they first added those in B/X), but there was a series of repeating 20s at the upper end of the table so you still had a 5% chance for a while against the best defenses, though at some point it stopped and you simply couldn't hit.
In 2E they formalized THAC0 (it had appeared in the monster appendix of the 1E DMG but wasn't fully integrated), which got rid of the repeating 20s, and turned it into a simpler number scale, but the mechanic was still using attack tables/matrices. Check my level and class against the matrix to see what score I need to hit AC 0, then roll my attack adding any applicable bonuses for strength or dex, specialization, race or class abilities with a particular weapon, magic weapon bonuses, etc.
Say for example that I have a 2nd Ed AD&D 2nd level Elf Fighter with a +1 longsword and +1 Strength bonus to hit. I also get a +1 to hit with Longswords for being an elf; it was a racial ability. Looking at the THAC0 table my number needed to hit AC 0 is a 19. I roll a d20 and add 3. Say I roll a 12. 12+3 = 15, subtract that from my THAC0 of 19, and I know I've hit AC4 (which happens to be the AC of Chainmail + Shield).
Compare to 5E. A 2nd level Elf Fighter has +2 Proficiency bonus with his longsword (this basically replaces THAC0), and probably a +3 either Strength or Dex bonus to hit (AD&D was stingier with ability bonuses to hit). Roll a d20 and add +5. Say I roll a 12. That's 17. I've hit AC17. Heck, Chainmail & shield is 18 because they made shields better! Oh well, missed by 1. Unless I had that longsword (or rapier) +1, but I didn't include it as those are rarer in 5E.