There are several types of 'prototypes.' Essays - Drawings and sketches showing the stamp design. Used for the initial approval process, they serve as a basis for the engraver to create the plates...
The normal name used is an 'essay.' Once it has been approved, there will be a plate created. That will be used to create 'proofs,' which are often used to create 'color trials.'
Originally, mail was stamped using ink and a stamp of some type. These stamps were made of wood, metal or in some cases, even potatoes! The impression indicated that the postage had been paid. When...