I found a similar postcard online and the back of the postcard contains detailed information about the auditorium.
Construction Date: 1925-1928
Seattle has constructed a Civic Auditorium, Exposition Hall and Recreation Field as a three-fold civic development, on a centrally located 12-acre site, bounded by Third and Fourth Avenues North, Harrison and Mercer Streets. The Auditorium, fronting on Third Avenue, is 194 feet by 274 feet in size, and comfortably seats 7,700 persons. Of these 5,000 are taken care of on the main floor and 2,700 in the balcony. It has a total of 40,000 square feet of utility space. The main floor is flat and of maple.
In 1959 construction began to transform the aging Civic Auditorium into a 3,100-seat Opera House in preparation for the 1962 Seattle World’s Fair. The remodeled hall was a fan-shaped room, with two balconies, seating about 3,100 persons.
Marion Oliver McCaw Hall is the latest incarnation of what was once Seattle’s Civic Auditorium (1928) which was then gutted and rebuilt as the Seattle Center Opera House for the World’s Fair in 1962.
VintageSeattle's Seattle Civic Auditorium Postcard
Marion Oliver McCaw Hall Article