Char-aina or literally the four mirrors. Four plates worn over a shirt of mail in Persia, India and Central Asia. Usually they are rectangular and the two worn on the breast and back are considerably larger than those worn at the sides.
There are several famous breastplates including the ones worn by Guru Gobind Singh. This particular one with depiction of Guru Nanak and Bhai Mardana is from the Howards Ricketts Collection.
Photos Courtesy of Freed.
Char-aina with depiction of Guru Nanak
Char-aina with depiction of Bhai Mardana