Bob, Mainship during that period may have been using the same systems as Silverton (sister company, at the time) so check to see if your dock water inlet is a Jabsco. If so, search pics of model 44410. If it looks the same, it's a 35 psi regulator incorporated as part of the inlet assembly.
If that's what it is... a more current Jabsco inlet/regulator is their model 44411-series, 45 psi, better. Unfortunately, not the same mounting footprint; slightly larger... but in our case that wasn't insurmountable. There are two versions, one with an L-shaped NPT connection on the back (44411), the other with a straight Snap-In (QEST) port fitting on the back (44412, oddly, not clearly listed on the Jabsco/Xylem site last I checked recently).
South River, Chesapeake Bay