Adding to your collection
Looking at the catalogue
An overprint is text or a design printed on top of a stamp.
Overprints are typically used in expedient or provisional situations,
where there is not time nor resources available to produce new types
of stamps. Examples include surcharges to reflect changes in postal
rates, changes of government, occupations, sometimes
The description of an overprinted stamp consists of a description of the overprint, its color (usually black), and a description of the stamp that has been overprinted, here called the base stamp. The base stamp is usually a type already issued, but in some cases the overprint is applied to unissued stamps.
The basic StampData assumption is that an overprint applies to a particular type of stamp - if an overprint was applied to several types, each overprinted stamp is itself a distinct type. However, in the process of development, it may be the list of types changes, and we want the overprint to follow along as the types change, or to flag that the base stamp may be any of several types. So we have a property onraw of an overprinted type that is a description of the base stamp, which ideally matches exactly one type, but which may match several types or none at all.
Many properties of an overprinted stamp derive from the base stamp, such as color, design, watermark, and perforation. If the base stamp is known, StampData automatically collects the base stamp's values for these properties and displays those for overprinted stamps, typically enclosed in parentheses as a reminder that they are inherited.