Plate Mail +1 is enchanted plate armor which offers an armor class of 2. Compared to normal Plate Mail, the weight of the armor has been reduced quite a lot, from 50 lbs to 20 lbs. The armor still has -3 armor class bonus against slashing attacks. There two different sets of Plate Mail +1, but these differences are only in appearance. This item appears in Baldur's Gate II: Shadows of Amn and Baldur's Gate II: Enhanced Edition. It's for sale at the Brynnlaw Store and sold by Therndle Daglefodd.
Aside from the aforementioned shops you can take it from Genthore in the Cloakwood Mines after killing him.
|“||Plate mail is a combination of chain armor with metal plates covering the vital areas such as the chest, abdomen and groin. Similar in construction to bronze plate mail, true plate mail comprises of chain and leather. In addition, this suit of plate mail is enchanted and gives additional bonuses to the wearer's armor class.||”|