Collector's Marks
Collectors have traditionally marked their drawings with stamps bearing their initials, monogram, or some unique design. Some appear on the recto, some on the verso. Some are stamped with colored inks, while others are simply impressed into the paper. These are called blind stamps. The custom seems to have been initiated, however, by auctioneers at the dispersal of collections. Estate stamps were also placed on drawings found in an artist's studio at his death.
 

by Michael Miller
Information supplied by: http://www.nyu.edu/classes