The Outlet Gauge screws into the Shutoff Assembly. It is useful for diagnosing problems with the High Lifter and the outlet line. Use the 300 psi gauge for lifts less than 500 feet. This is a very nice gauge which is glycerin-filled to resist pressure shocks and corrosion. It has a 1/4" pipe thread. It has a 2" face diameter and the mounting threads are on the bottom of the gauge.