The WWI war rates for postcards and letters were active until July 1, 1919.
This is an example of a 2-cent coil stamp (Scott #492) used on a postcard July 7, 1919, after the rate was changed back to the pre-war amounts.
(Some of the taxes on mail, such as Parcel Post were rescinded later than that.)
The postcard would have been fine with only a 1-cent stamp. The postcard is cancelled by the American flag machine of Olympia, Washington.
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