Well, they're both item from the BGII items mod for IWD. The Drow Chainmail and the Harper's Pin. The Harper's Pin effects you as the shield spell, but it doesn't stack with the Chainmail. When I remove the Chainmail, or equip the Pin on someone else, it gives its shieldbonus.
Here's the mod:
Icewind Dale 2 Mods It's the second one.