Maxperson
Morkus from Orkus
Yep. The 1e DMG is the only one where the layout is worse than the 5e DMG.The 1e DMG organization layout makes this some work to find it.
The 1e DMG combat section starts on page 61 with the attack tables on page 73. The rule explanations on attack modifiers is on page 28 in a noncombat section on Armor, Armor Class & Weapons between sections on money and hirelings. So look for the reputed magical properties of gems and then keep going and you will find the AC stuff about 30 pages before the combat section.