There are no mint marks on all U.S. coins 1965, 1966 and 1967. And there's no way to tell if they were minted in Philly, Denver or San Fran. People were starting to hoard the silver coins and all the mint mark coins. And the gov't's brilliant idea to discourage hoarding of MM coins was to take off the MM. Starting in 1968, they put the mint marks back. Philly never had a mint mark until 1979 Susan B Anthony and then 1980 on all other coins except the Lincoln cent. As a special anniversary for the US mint, they put a P on the Lincoln cent in 2017, and then removed it. But it's on all the other coins now.
There are a few coins ( mostly cents and dimes) that came in proof sets, and are supposed to have an S but they have nothing. These are very rare and valuable, and look nothing like a regular circulated coin.
