It has to do with the OSAC timing . My Coronet (1972) has that same piece ; and I disconnected it when I disablled that OSAC ( stupid thing ! I don't need for the timing to be even more inherently-retarded ! ) .
On a '73 model , that's where the OSAC valve was placed ; '74 & later the OSAC was placed on the air cleaner assembly .
