As you say, her birth certificate would be the most reliable info, but that is not likely available, given the year.
Other records which can help provide circumstantial evidence would be:
-Her death certificate - you can order it from the state of Washington, but if her grandson was the closest survivor, he may have not known her parents names
-The death certificates of her children may list her maiden name.
-Obituaries for her parents or the Alvords. You can browse the California death index for 1905-1939 at familysearch
Here is the page for the Alvord'shttps://familysearch.org/pal:/MM9.3.1/TH-1971-26179-9659-52?...
Henry Alvord died on January 25, 1918
Marcia Alvord died on April 4, 1919
There may not be obits in the LA paper, but you may get lucky.