Everything you never knew you wanted to know about the Mercury Project
Cabin air outflow valve
From RixWiki
The cabin air outflow valve, located on the forward pressure bulkhead provided for allowing the cabin to vent to the outside atmosphere after re-entry. It was of the same basic construction and operation as the cabin air inlet valve.
The design was changed and documented in the February 1962 edition of SEDR-104. The early design used a snorkel valve, while the newer design used a diaphragm flapper ventilation valve.
