The commemorative half dollars for the 1915 Panama-Pacific International Exposition were struck at the Branch Mint in San Francisco, but the legislation authorizing the gold and silver coins for the Exposition included a special provision that would have allowed the silver half dollars to be "coined or finished and issued from the machinery to be installed as a part of the exhibit of the United States mint at said exposition, and for the purpose of maintaining the exhibit as an educative working exhibit at all times the coins so minted may be remelted and reminted."
The law authorizing the coins allowed for up to 200,000 half-dollar coins to be struck, but only 60,000 were prepared by the San Francisco Branch Mint. The minting of just 30% of the authorized maximum was likely due to another special provision included in the law for the half dollar: "All of said 50-cent silver coins herein authorized not issued to and at the request of said Panama-Pacific International Exposition, whether the same are coined as a part of said working exhibit or coined at the mint in San Francisco, shall be remelted upon the official closing of said exposition." Without a good sense of potential sales for the half dollar during the limited run of the Exposition, it was a prudent move to not strike the full allotment of the authorized coins. (Final sales figures for the coin would prove just how prudent the decision was!) Read More: Commems Collection
The Exposition ran from February 20 to December 4, 1915. Farran Zerbe, the head of the Exposition Company's Department of Coins and Medals, announced in April 1916 that the small number of coins remaining in his/the Exposition Company's possession would be available for sale until about May 15, 1916, at their original Exposition prices, after which, they would all be returned to the Mint for melting.
Though the law authorizing the Panama-Pacific coins did not include language requiring the immediate melting of the gold pieces, per Mint reports, 3,251 of the gold quarter eagles, 855 of the round $50 gold coin and 1,017 of the octagonal $50 gold coin were returned and subsequently melted in October 1916; per Zerbe, no leftover gold dollars needed to be returned. A total of 32,866 of the silver half dollars were melted during separate sessions in September and October 1916 - this left a net final mintage of 27,134 (a far cry from the 200,000 authorized and less than half of the count of the first batch struck!).
So, unlike many of the other programs that followed, the Panama-Pacific Exposition commemorative coin program had a quick and tidy conclusion! Kudos to Farran Zerbe!
Here's my example of the half dollar; the coin is a brilliant and flashy example for which my scanner has "washed away" its luster.