The 3326 and 3327 are normally closed relay valves, used to sequence air flow based on pressure levels.  This allows the designer to interlock control functions, or allow for time delays in reversing without damage to sensitive pneumatic or propulsion equipment. The 3326 has a single pilot-to-shift input, whereas the 3327 has two pilot inputs to permit pressure return of the spool. 

The 3326 / 3327 valves are available in a variety of light to heavy duty spring configurations.  Constructed of bronze and stainless steel, these units have 1/4" NPT ports and are rated to maximum supply pressure of 200 PSI (13.8 BAR). Photo shown is of a 3327.