The purge port on the exterior of the hatch.
After the Apollo 1 fire, the atmosphere inside the command module was changed
from 100% oxygen to a sea-level pressure mixture of 60% oxygen and 40%
nitrogen, which reduced the fire hazard. The pressure
and composition of the cabin gas during the prelaunch period was controlled by
ground support equipment through the purge port.
During launch, a pressure relief valve vents the excess gas
overboard. During normal space operations, cabin
atmosphere was pure oxygen and the pressure was maintained at 5 psi.