Traditionally, collectors have stamped all the verso and sometimes the recto of prints and drawings which came into their collections. These are known as Collectors’ marks, though galleries who act as intermediaries between the artist and the collectors have also used similar stamps.
When you know how to recognise these stamped marks, you can also identify the collectors who once possessed the art. Hence the marks help establish or support claims of the provenance.
However, a Collector mark should be regarded with caution and skepticism. A mark can be defrauded in order to create instant provenance that otherwise would not exist or it can fill out inconvenient gaps in the lineage of art collectors who have acted as custodians. The latter sneaks in a white lie in order to maximise the market value of the art.