Guam
In 1899, the local postage stamps were overprinted "Guam" as was done for the other former Spanish colonies, but this was discontinued shortly thereafter and regular U.S. postage stamps have been used ever since. Because Guam is also part of the U.S. Postal System (postal abbreviation: GU, ZIP code range: 96910–96932), mail to Guam from the U.S. mainland is considered domestic and no additional charges are required. Private shipping companies, such as FedEx, UPS, and DHL, however, have no obligation to do so, and do not regard Guam as domestic.

Q: In what year was the local postage stamp overprinted and has since been replaced?
1899