The Mason? Is it forgotten already? It opened in 1903 as the Mason Opera House and was demolished without much attention in 1956 for a State of California office building. The location was 127 S. Broadway, 2 blocks north of the Million Dollar.
See my page on the Mason Theatre for some photos I've come across recently as well as stage specifications I found in the 1906-07 edition of Cahn's Theatrical Guide, a reference manual for traveling showmen.
The Mason had a great career as a legit house and film emporium. It was the home of Mexican language pictures and stage shows for years under the management of Frank Fouce.
During the late 30s and early 40s it was home to a number of Federal Theatre Project presentations. The poster shown here is a 1938 item in the collection of the Library of Congress.