This stamp was found in a mint supply of the 50¢ Altoona (PA) local real estate transfer tax stamps. It graces the cover of an updated catalog of these stamps edited by Rob Conley and published by the State Revenue Society. If you collect state and local revenue stamps you need to be a member of the State Revenue Society. They also have recently published a State Revenue Catalog edited by Dave Wrisley. Both of these publications are first rate and will guide us toward new discoveries as we organize our holdings of these fascinating stamps.
