Shown here is the front of the Castro house, a typical adobe structure of the 19th Century Mission era. The long balcony is suspended primarily by roof beams from the first floor which extend out past the inner walls.

The description states:
CASTRO HOUSE
San Juan Bautista, California
This historical house, dating from times when California was a part of Mexico, is restored and open to the public.
–The postcard likely dates from the 1960s.