Stiletto

The stiletto is a type of narrow-bladed knife, sharp only at the point. Its most unusual trait is that it confers a +2 (non-magical) bonus to attack rolls against certain armor types: Plate mail (bronze and normal), ring mail, and chain mail. (This is because its narrow point and blade slips in more readily through any sort of armor that is not solid metal or overlapping plates of metal.)

It otherwise behaves like any other knife, and Knife weapon proficiency is exactly the same as Stiletto weapon proficiency: If you know one, you know the other equally well, at no additional cost in proficiency slots.