Have you ever looked at the note to collectors the US Mint included in the 1965 Special Mint Set?
It states:
Bureau of the Mint, Treasury Department, Washington, DC 20220
Dear Friend:
These coins, which constitute our first issue of United States Special Mint Sets, were struck at the U.S. Assay Office at San Francisco, California. In addition to the 1965 dated nickel and cent pieces, the set contains three new clad coins authorized by the Coinage Act of 1965.
Special Mint Sets are made from specially prepared and polished blanks and struck on high tonnage presses with polished dies. We have endeavored to minimize scratches and discoloration. It is possible that tarnishing and discoloration may occur, particularly if removed from the packages and if exposed to heat or direct sunlight. We cannot undertake to make exchanges.
Your name has been added to the mailing list kept by the United States Assay Office at San Francisco an you will be sent order blanks for such new issues as may be offered for sale.
We hope these coins will be pleasing to you, and that we may have the privilege, again, of serving you.
DIRECTOR OF THE MINT
Neither Proof Sets nor regular Uncirculated Mint Sets were minted or distributed in the years of 1965 through 1967. Instead, the Special Mint Set was struck and distributed out of San Francisco.
Interestingly, the Mint in San Francisco had previously ceased operations, consequently the name had been changed to Assay Office. However, with the mintage operations back in production mode, its name would change back to the San Francisco Mint.
From the Treasury’s web site detailing their production facilities:
The San Francisco Mint — The San Francisco facility was authorized by the Act of July 3, 1852. It first produced coinage in 1854. Coinage operations were suspended in March 1955, but the plant continued to operate as an Assay Office. Public Law 87-534, passed on July 11, 1962, changed the official designation from “Mint to Assay Office.” Coinage operations were again authorized under the authority of the Coinage Act of 1965, approved on July 23, 1965. The San Francisco Mint:
Manufactures and packages Regular and Silver Proof Coin Sets;
Manufactures, packages and ships Commemorative Coins
On another note, the Coinage Act of 1965 included the change from silver halves, quarters and dimes to clad. Also, the Coinage Act removed any distinguishing mint marks from the coins in 1965, 1966 and 1967. When the mint marks resumed in 1968, they moved to the obverse of the coins.
You can find more about the coinage act on the Mint’s web sites here and here with discussions about the mint marks. The Treasury Department also includes more about the Coinage Act in the 1900 to Present Timeline.