Hydrogen Gas Actuator
This is the cylindrical part that resides at the top right part of the [/equipment/plans/elite-proton-pack-plans/ Proton Pack], just above the Gun Mount. Its purpose is unknown, but may coincide with the [/equipment/proton-pack/part-names/crank-generator/ Crank Generator] to which it is attached.
The HGA has four 1/4" socket head cap screws mounted to its face around a relatively thick metallic label. Prior to obtaining higher-resolution photos, it was thought that a screw was in the center of the label. This has since been shown to be non-existent. There is also a [/equipment/proton-pack/legris/ Legris] 4mm elbow and a [/equipment/proton-pack/clippard/ Clippard] L-fitting and hose barb (#15090-1 and #11752-1, respectively) mounted to the side. The angular orientation of these two parts differs from pack to pack:
Bolt and fitting configurations can be seen here, if one disregards the center screw that is specified: http://www.gbfans.com/images/equipment/plans/stefan_8.jpg