I don't know of any hard facts on the # of Argosy 20 motorhome production numbers. Others had tried looking at the Airstream serial number but I think there are Argsoy 24', 26', and 28' units mixed in the sequence. There is another thread in the motorhome section about posting your serial # (not the chassis VIN).
Most of the 1974 units are the front bath. After 1974, the rear baths make up the vast majority of the Argosy 20 units. 1977 is clearly the largest production year for the Argosy 20s. All most all of them were rear bath units.
Here is a link to my Argosy 20 page