Plaquet

To the back-piece of the cuirass for the protection of the loins, was hooked on a piece of armour, called Garde des Reins, or culettes; and the breast-piece was occasionally strengthened by an additional plate called a Plaquet.

[…] The plate of steel used as additional defence, and specifically the doubling of the lower part of the breastplate; a similar plaquet was used for the back.