This is the cylindrical part that resides at the top right part of the Proton Pack, just above the Gun Mount. Its purpose is unknown, but may coincide with the 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 Legris 4mm elbow and a 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
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