The "square" body format was pioneered by Argosy before making its debut in the Airstream line. The first year for the "square" bodied Argosys was 1986. There is a fair amount of information about the Argosy lineage on the Airstream corporate site at:
Airstream and Argosy History
While the 1988 Argosy isn't listed in the table of weights and measures at the Airstream corporate site, the 1987 Argosys are listed. While it might not necessarily be true for all models, but there is usually a fairly close weight relation between two Argosy trailers of similar sizes when they are separated only by a year or two. The weight table can be accessed at:
Airstream and Argosy Trailer Weights
The "square" Argosys as well as a few "square" Airstreams surface from time to time. In fact, there was at least one "square" Airstream trailer at last year's WBCCI International Rally in Sioux Falls, SD.
In looking at the information in the chronological history of Airstream and Argosy, I noticed that 1988 was the first year for an Argosy fifth wheel - - I can't remember ever seeing one of these - - I have seen several of the Airstream Land Yacht fifth wheels, but can't remember ever having seen an Argosy fifth wheel.