One other interesting observation. The gold is absolutely worn off the front trigger but still bright and shiny on the back trigger, as if the back trigger was rarely used. Which leads me back around to singles and handicap trap.

John, contact the seller and find out the name of the owner in the last 50 or so years. That may be the name you can submit to Del Grego for documentation of the barrel work. As others have guessed, it could be Del Grego or Frank Lefever and Sons. The rib looks very professional, but not Parker factory. I'm guessing Lefever.