This is a power plug received in shipment with the original Spartan flight instrument carried into space five times in the Shuttle payload bay and deployed for the duration of each mission into a parallel orbit with Shuttle. The SPARTAN program was created by NASA in the 1980s to replace its suborbital sounding rocket program.
NASA Goddard transferred this to the Smithsonian in 2004.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.