I believe that is incorrect: my understanding is that the non-Chainmail rules in OD&D used some modified naval combat rules, which became standard D&D combat. The ships in these rules were put in "Armor Classes": First Class was hardest to hit, Second Class was slightly easier, etc...
In Chainmail Appendix B (of my only surviving copy, 3rd edition, TSR 1975) AC on the Man to Man Melee Table went from "No Armor" on up going right (no numbers) to "Plate and Shield" with separate categories for Horses (Unarmored or barded)
The results gave you the number on 2D6 needed for a "kill" / Hit (in later D&D). No numbers for the armor types. The table matched Weapons vs. Armor. Just below that the "Individual Fire with Missiles" table ran from "1" on up to "8". presumably from No Armor to Plate and Shield. Horses were still listed as separate (no numbers). I think they numbered the AC on the second table to save space as each result listed three values based on range. Mind you they didn't explain which # was what armor from the above table, they just assumed you would know
Again it was Weapon vs. Armor on the table. There were also tables for fantastic creatures vs. other fantastic creatures (including Heroes and Superheroes) that ignored armor and were for mass battles (which was what Chainmail was about after all).
Original D&D listed characters fighting ability by their equivalent in "men" or as a Hero / Superhero / Wizard etc from Chainmail with increasing ability as you leveled up. D&D directly referenced the system in Chainmail as being the one used for combat. Hits did 1D6 damage for all weapons as oppose to a "kill" with suggested variations for daggers and two handed swords iirc (-1 and +1 I think). And it didn't provide the system. That was in Chainmail and they assumed you would have a copy (we did, we were miniature gamers)
They also provided an "alternate system" involving character level vs. Armor Class with Armor Class ranging from 9 "No Armor or Shield" on down to 2 "Plate Armor and Shield" that most players would recognize today from 1E AND 2E (although no armor became AC 10 in AD&D). Well, probably recognize it anyway. Book 2 of Original D&D gave monster stats including AC, Hit Dice etc. We switched to the alternate system for combat pretty quickly (using chits numbered 1-20 as we took a while to acquire those odd 20 sided dice) because it meshed with the monster stats and included the effects of character levels and classes. By the time the first D&D supplement (Greyhawk) came out in 1975 I think the "alternate" system was the assumed normal one. And Greyhawk introduced thing like different damage ranges for various weapons and sizes of opponents too! The two handed sword became *the* weapon to use against larger than man sized opponents (3-18 damage)...
If you wonder why there was so much innovation and homebrewing in the early days this might tell you why. It was all pretty much like this. Undefined, unorganized, and totally fantastic. It was a great time to be a gamer
*edit* For memory, grammar, etc.