Private overprints have been used for a number of reasons, though generally this use makes the stamp or stamps invalid for prepayment of postage.
In Britain, privately overprinted stamps served as, among other purposes, receipts for tax payments.
In Italy, after the fall of Mussolini and his establishment of the Italian Social Republic, stamps of the King were overprinted with fasces by Fascist sympathizers.
Private overprints have also been used in the United States. A private overprint exists on the Florida commemorative, and during the Vietnam War, a woman privately overprinted the stamps on her outgoing mail with the slogan "Pray for War" before postal authorities compelled her to move this stamping away from the stamps.
See also:
underprints, perfins
External link
http://www.giorgiobifani.net/rsi15a_eng.htm