A ceramic applique seperation shield from a Huey Cobra's turbine shielding, I think. It's about the right size, or possibly armor augmentation plate for the Humvee made of similar materials, purpose-built to fit the size and application required.
Since the fabric carriers for such plates have to be custom-fitted, plates originally meant for other applications can be quickly adapted to newer uses by simply dropping them in a new nylon carrier, and attaching them for use. It's real hard to tell what quality and grade plate is inside, until you pull the plate out and check.
I once received two composite ceramic clipboards for testing, set one up and blasted away at one with hot .41 magnum and .357 pointed-bullet loads, none of which made it through. Duly impressed, I carried the second on a more-or-less daily basis for most of a decade.