I think you're right about the location of the entrance on our mystery building,
e_r.
Here's a close-up of those "For Sale" signs. The one on the left belongs to R A Rowan at 419 Douglas Building. The one on the right shows the distinctive diagonal design of W M Garland & Co. Both companies moved around quite a bit - the 1901 CD has both of them at 212 Wilcox Building. I believe the Garland sign here says "Wilcox Block". The 1906 CD lists R A Rowan at 200 Herman W Hellman Building.
Detail of picture in
USC Digital Library
Garland's diagonal design can be seen in color on the front of the 1917 CD (1920 refers to the year of the population estimate). Note the address in the Van Nuys Building.
LAPL