Dia Ring + Dia Armor combo

[GM-Pandora]

+ % ATK bonus works like race card which is why you don't see the bonus directly in your stats, here's the item script taken from rms:

bonus2 bAddRace,RC_NonBoss,3; bonus2 bAddRace,RC_Boss,3;

What this means is 3% more damage on non-boss monster and 3% damage on boss monster, nonboss + boss together equalling all monsters.