This is an unflown example of a Stablized Platform, which was used to guide and control the Saturn V rocket. It contained within it a set of gyroscopes and accelerometers, which maintained a fixed position with reference to the Earth's gravitational field, as the rocket ascended from its launch pad.
Markings on the device indicate that it was modified in March, 1971, when Saturn rockets were still being used to launch missions to the Moon. It was probably intended for a lunar mission, but it was not flown.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.