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The sign accompanying the ablated heat shield sample.

It reads

Ablated Apollo Heat Shield Sample

This is a piece of a heat shield from an Apollo Command Module flown on a suborbital test in 1966. The segment shows the steel base and how the filling material was eroded (ablated) into a grey ash by the intense heat of reentry.

Transferred from NASA

Although it doesn't mention the specific mission, "a suborbital test in 1966" would indicate that it's from AS-201 (February 26, 1966) or AS-202 (August 25, 1966). (AS-203 was flown on July 5, 1966, but was an orbital mission which did not carry an Apollo Command Module.)