This model also exicted in 1964 but that year, it was a Skylark Sports Wagon and there were Skylark scripts on the body instead of the SportWagon scripts (with more or less space between the Sport and Wagon...) of the 1965 newer models. Buick documentation such as production car orders and advertising usually spells it Sportwagon in a single word and some 1964 documentation shows it spelled in two words to the plural:
See (1964) Skylark Sports Wagons
1964 Skylark Sports Wagon
1965
Sportwagon ad
1965 SportWagon emblems
1966 ad