The first US mint was built in Philadelphia and coins mint there did not have mintmarks until 1980, except for wartime nickels which did have mints marks, and some error coins that did not.(see Sandusky Dime)

US Mint
P- Philadelphia, PA

Branch Mints
C - Charlotte, NC (gold only)
CC – Carson City, NV
D – Dahlonega, GA (gold only 1838 to 1861)
D – Denver, CO (1906 to present)
O – New Orleans, LA
S – San Francisco, CA
W – West Point, NY

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