You see the beginnings of metal breast and back plates sewn onto leather and fabric armours depicted in MSS like the 'Shah Nama' around the 1540's. The current thinking is that the separate entities known as chahar aina 'four mirrors' start to appear in Turkey as part of a sort of American footballer style upper torso harness worn over a mail coat in the late 16th century. The earliest dated 'char aina' is 1702 in the Royal Scottish Museum, Edinburgh.
They may have been a response to the development of early firearms, or nastier 'tin opener' style anti-armour weapons like pollaxes, tabarzins and war hammers which had spikes capable of piercing mail if used with some force.