A mark on a document by which a company offical endorses the document to be used in a specific way (classification) or its contents and purpose having a specified status (authorised/approved... or not) could be called an “endorsement mark”.
It may appear in the form of a stamp and be called “signature stamp” (e.g. http://en.wiktionary.org/wiki/гриф ), however note that this version may be misunderstood as merely a signature made by means of a stamp without a specific endorsement.
Therefore, “endorsement mark” (regardless of whether it is stamped, digital, handwritten or what details it includes).