From the looks and location of it, it is the MAP sensor. This is responsible for sensing manifold vaccum (in effect sensing engine load). In combination with a MAF for fueling, this is also used in modern vehicles to check functionality of EGR valve when the throttle is released.
EDIT: With CNG, it can be plugged to emulator to check at what load engine is running so as to advance the spark as per load curve in emulator.