The idea was invented by Perkins Bacon who used it to print stamps for Trinidad (1851), Barbados (1852) and Mauritius (1858), all featuring the same Britannia design.
This method has the advantage that most of the design remains the same in each variant of a stamp series with only the values, names and colours changing.
Key plate stamps were used extensively by Great Britain, Germany, France, Spain and Portugal.
[2] Key plates were also a ubiquitous feature of revenue stamps of Burma/Myanmar, Great Britain, India, Ireland, Malta and Pakistan.
These had a tablet at the bottom, and this was appropriated (overprinted to indicate the type of use), e.g. Consular Service, Contract Note, Notarial, Special Adhesive, Stocks & Shares.