This bunker is located above the pacific ocean at a part of the coast called "devil's slide" few miles south of San Francisco. I want to thanks Kevin L. for sending me this tip, this is truly an amazing location. I had the chance to go there and take some pictures, I can tell that the name "devil's slide" is perfectly appropriate!
There are 2 sets of bunkers in pretty much the same location, the lone bunker to the south, built originally on a thick pillar of sandstone. The stone has eroded so much that the building hangs over it on all sides. And some more to the north.
The following description from the United States Army Corps of Engineers website refers to those constructions (the one to the north) : "Prior to 01 July 1940, the War Department acquired 9.61 ... acres for a triangulation station and observation site. Devil's Slide was one of a series of observation posts during pre-radar days and was a part of the Harbor Defense of San Francisco. Military personnel would use binoculars and compasses to search for ships at sea and relay the position ... using information received from other observation posts. Improvements to the site began in 1943, and included three observation pill-boxes, one electric generator bunker, one communications and command bunker, and an observation tower."
The south bunker.
Looking to the north.
The 3 other constructions north of the first bunker.
south bunker coordinates : 37°34'05.23"N 122°30'58.99"W
north bunkers coordinates: 37°34'28.43"N 122°31'09.04"W