At the insistence of the Mercury astronauts, an egress hatch affixed with explosive bolts was added to the one-man Mercury capsule to allow a quicker emergency exit from the spacecraft. It was first flown on Virgil I. "Gus" Grissom's "Liberty Bell 7" during the Mercury-Redstone 2 suborbital mission in July 1961, and jettisoned accidentally after landing, resulting in the sinking of the capsule and the near-drowning of the astronaut. All further missions using the hatch had no problems, however.
This particular hatch lacks external shingles and has been modified to carry a micrometeoroid experiment built by Bendix. It never flew in space. In 1973 the NASA Manned Spacecraft Center in Houston transferred it to the Smithsonian.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.