The (back of) flyer promotes this machine as "Convertible to Add-A-Ball" but no mention of option in the 1973 parts catalog and it is a later genre than the added ball kit (relay) method for Rocket III, MiniZag etc.
As MarkG says the machine has an Extra Ball RE enabled via the high score settings plug in lieu of Replays. Do you have the labels marked on your settings plugs? The Jones plug wire is Yellow/Red.