Private overprint
In philately, private overprints or commercial overprints are overprints applied to postage stamps, postal stationery or revenue stamps by anyone other than the official stamp-issuing entity. These overprints have principally been used as a security measure, however, propaganda and commemorative examples are also known. When overprinted for security purposes, they serve a similar function to perfins. It is important to distinguish between private overprints and private cancellations.
Background
Private overprints have been used for a number of reasons. Generally they cannot be used as control marks, as the stamp or stamps are thus rendered invalid for prepayment of postage. However, while in Great Britain privately overprinted stamps usually served as receipts for tax payments, in 1859, upon application of the Oxford Union Society, there was a unique instance of a private overprint being approved for application on the face of stamps. This is to be distinguished from the more or less frequent application of private overprints to the back of stamps as control marks.In Great Britain, commercial overprints were principally used to pay the tax on receipts and are found on both revenue and postage stamps with a great number of different varieties known between the 1880s and 1970s.
Other countries which had commercial overprints include Canada, Ceylon, India, Kenya & Uganda, New South Wales, New Zealand, South Africa, Spain, the Straits Settlements and Victoria.
Private overprints can also originate from speculative philatelic purposes produced deliberately with a view to selling them to unsuspecting collectors.