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