I noticed that Buick Special Deluxes were all listed as "Special De Luxe", which is only correct for the 1950s version. It appears that the 1961-69 mid-size models were actually "Special Deluxe".

http://imcdb.org/vehicles.php?make=Buick&model=Special+De+Luxe&modelMatch=1&modelInclModel=- on

Brochures of the 1960s model:
http://oldcarbrochures.org/NA/Buick/1961_Buick/1961_Buick_Special_Brochure/1961-Buick-Special-11
http://oldcarbrochures.org/NA/Buick/1969_Buick/1969_Buick_Prestige_Brochure/1969-Buick-Prestige-56-57