Actually, its a piece of Shinano’s armor that was recovered with part of Shinano’s main battery turntables that were never installed due to Shinano’s conversion, late in the war, to an aircraft carrier.
The sample was subjected to a point-blank 16” Iowa-type shell test. The nature of the test, given point-blank distance and lack of proper “falling angle” foir the shell made the armor likely fail easier than it would have in real action.
But there is no doubt that it was not immune to the USN’s highly effective and deadly 16” armor-piercing shells. That piece - along with an 18.1” barrel intended for SHinano’s main bateries, which recovered at the same site as the armor pictured above, are on the grounds of a Naval facility near Washington DC as memory serves me ....