The "Architecture and Archaeology" series also included a number of larger format airmail stamps (1950–1976) with common design, although some specialists treat the two series separately. The airmail series had even more varieties than the surface mail stamps, some of which are fairly rare, selling for over $100 each. Pulver has stated that this issue "is made to order for a modern specialist, with collecting challenge comparable to a 19th-century classic issue." Some of the later issues in this series were printed on light-active paper, coated with optical brighteners causing it to fluoresce under ultraviolet light, or which had phosphorescent tagging added. These coatings were used for security purposes and to facilitate high speed machine sorting.