This is the PAM (Payload Assist Module), a portable launch platform for helping launch small satellites from the Space Shuttle than would be normally possible since the Shuttle only operates in low Earth orbits. The PAM has a spin motor and spring device for spinning the satellite to create inertia for stability and for ejecting it.
This PAM-D was flown on four Space Shuttle missions including STS-21 and STS-25, both in 1985. The latter flight carried the Arab Shiek S. Al-Saud and Patrick Baudry.
Sheik Al-Saud was a space tourist while Baudry was the first Frenchman into space, On the same mission the ARABSAT was launched. The PAM-D also launched ANIK-C2 in 1983 (STS-7), Weststar-VI in 1985 (STS-41B); and AUSSAT-B in 1985 (STS-27).
This object was donated to the Smithsonian in 1990 by the McDonnell Douglas Astronautics Co.
McDonnell Douglas Astronautics Company