I have been visiting this place for over 40 years, and it's only been a few months since I first noticed this plaque. Goes to show that you can miss a lot of things if you're not paying attention.
Victor Ramón Castro was a member of the Castro family for which many places in the Bay Area are named. His father Francisco María Castro came north with the De Anza expedition in 1775 and was a soldier at the San Franciso Presidio before becoming one of the founders of the Pueblo of San José. With his brother Juan José Castro, Victor was grantee for the Rancho El Sobrante in 1841. After California statehood, he was elected a Contra Costa County supervisor in 1852. Nearby Point Isabel is named for his daughter.
Bring yourself to the above coordinates to read the plaque and deduce the coordinates for the actual cache, which is within walking distance within the plaza. It is not behind any fence or wall and not within any structure.
UPDATE 6/22/17: The cache has been moved to a new but nearby location, and the description below has been modified to produce the new final coordinates.
The cache is located at N 37 5A.BCD W 122 1E.FGH, where:
A = 3rd line, 3rd digit
B = sum of three digits on next to last line
C = 5th line, 2nd digit
D = 6th line, 7th digit
E = 7th line, 2nd digit
F = 5th line, 7th digit
G = last line, 4th digit
H = 8th line, last digit
Watch out for vehicular traffic as they are not watching for you (remember what I said about not paying attention), and cross any streets or parking lots with due caution.
Use appropriate stealth when you search for, retrieve, and replace the final.