This is a Project Mercury retrograde rocket package built by the Thiokol Chemical Corporation, which provided reverse propulsion to slow down the one-man Project Mercury space capsule prior to reentry through the earth's atmosphere.
Like the spacecraft's escape, tower jettison, and posigrade rockets, the retrograde rockets used solid propellants because of their simplicity and reliability. Each rocket, which produced about 1,000 pounds of thrust for ten seconds, fired independently of the others in five second intervals to decrease the capsule's velocity. To ensure safe reentry, only two of the three motors had to function.
The circular retro package sat underneath the heat shield and jettisoned a minute after engine burnout to afford unencumbered capsule reentry. The retro rockets could be fired automatically by an orbital timing device or manually by the astronaut, if required.
NASA transferred this rocket package to the Museum with the Mercury #19 capsule in 1968.