The 1925 California Diamond Jubilee Half Dollar was struck to commemorate the 75th Anniversary of the admission of California into the Union. It had an authorized mintage of 300,000 coins, but just half this figure (150,000) were struck at San Francisco in August 1925. Ultimately, ~63,600 coins were returned to the Mint to be melted, leaving a net mintage of ~86,400.
The "S" mint mark can be found on the coin's reverse below the "D" in "DOLLAR".
The first coin of the mintage, struck on August 12, 1925, was presented to Senator Samuel Morgan Shortridge (R-CA); Shortridge introduced the California coin bill that became law (after multiple amendments).
Samuel Morgan Shortridge - Circa 1922
(Image Credit: Library of Congress, Prints and Photographs Division. Public Domain.)On Monday, August 17, 1925, Shortridge was presented with the first California half dollar struck while he was in San Diego; the presentation was made by Michael Kelley, Superintendent of the San Francisco Branch Mint.
The "First Coin" honor typically went to a Very Important Person ("VIP") connected to the coin program; multiple United States Presidents have been recipients after signing the respective coin bill into law. (Search for "First Coin" under "US Classic Commemoratives" within the "Commems Collection" list for other stories.)
1925-S California Diamond Jubilee Half Dollar

For other of my posts about commemorative coins and medals, including more California half dollar stories, see:
Commems Collection.