this 1973 unit is equipped with three prong
Gary, it is unlikely that Phase Linear used a three-prong plug on an amplifier back in 1973, but there are people on this Forum that would know.
More likely it was an upgrade made by a previous owner, possibly to use as a stage or studio amplifier. Most stage electricians and all studio electricians demanded that all electrical equipment be grounded to prevent hum or electrocution.
Not sure what "two junctions for output" means... How about a picture of that capacitor that you mentioned.
I always run a new Ground wire from the Capacitor Buss (STAR Ground) to the Watts Abundant Board. Why not, wire is cheap.