The January-February 1947 issue of
California Highways and Public Works contains the announcement "Erection of Signs and Notices Designating California Freeways":
https://archive.org/stream/californiahighwa194547calirich#page/n517/mode/2up
While the "Freeway - Access Rights Restricted" sign is well-known and still widely seen, the article also describes a concrete markers that would be used to mark access openings in a freeway:
QuoteOn freeways where the abutting property has a limited access right, the outer boundaries of such access openings will be marked by concrete monuments similar in design to Illustration C. These marker posts will be located within the highway right of way immediately adjacent to the right of way line at the outside limits of the access opening, with a plus (+) sign on the monument facing the space included in the permitted access opening.
(https://i.imgur.com/rYLrdxk.png)
This appears to be different from the "C" right-of-way markers. Did these markers ever get erected? And if so, do any of them still exist?