A number of Los Angeles city directories are available online at the LAPL--25 or so from 1909 to 1987--but, unfortunately, none have a reverse-lookup section for telephone numbers. In fact, the ones from 1909 to 1942 don't list phone numbers at all, only addresses. The first directory available online after 1942--the 1956 issue--is organized by address and has phone numbers, but still has no way to look up by phone number.
As for GLadstone, I think this might have been a Malibu exchange.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Here's a Mullen-Bluett store I don't remember seeing a picture of before. It was at 6316 Hollywood Blvd near Vine.
To its left is R. H. Dyas, bought by The Broadway ca. 1931--the building which still has the iconic Broadway-
Hollywood sign on top. Pic is dated July 15, 1929.
LAPL
Another shot new to me--the shop of the famous Travis Banton at 6530 Sunset:
LAPL
This one, mercifully, is still there, as the Cat & Fiddle Restaurant.
Google Street View