Due to a small change shortage that was erroneously blamed on coin collectors, Congress eliminated the mintmarks on all coins from 1965 through 1967. However, that doesnt mean that everything was struck in Philadelphia. For instance, all of the business strike 40% silver halve were struck in Denver. The Special Mint Set coins were all struck in San Francisco. In the same vein, Congress froze the date for silver coins in the Coinage Act of 1965. The mints struck 1964 dated silver dimes, quarters, and halves until 1966. Things caught up in 1967.
Look at this: https://www.usmint.gov/learn/history/historical-documents/mint-marks-restored-to-coins