countermark

from The Century Dictionary.

• noun A mark or token added to a mark or marks already existing for greater security or more sure identification, as a second or third mark put on a bale of goods belonging to several merchants, that it may be opened only in the presence of all the owners; specifically, the mark of the Goldsmiths' Company of London, added to that of the artificer, to show the metal to be standard.
• noun A small device, inscription, or numeral, stamped upon a coin subsequent to its issue from the mint.
• noun An artificial cavity made in the teeth of horses that have outgrown their natural mark, to disguise their age.
• To add a countermark to, in any sense of t hat word.

from the GNU version of the Collaborative International Dictionary of English.

• transitive verb To apply a countermark to.
• noun A mark or token added to those already existing, in order to afford security or proof; as, an additional or special mark put upon a package of goods belonging to several persons, that it may not be opened except in the presence of all; a mark added to that of an artificer of gold or silver work by the Goldsmiths' Company of London, to attest the standard quality of the gold or silver; a mark added to an ancient coin or medal, to show either its change of value or that it was taken from an enemy.
• noun (Far.) An artificial cavity made in the teeth of horses that have outgrown their natural mark, to disguise their age.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A mark or token added to those already existing, as a security or proof.
• noun An artificial cavity made in the teeth of horses that have outgrown their natural mark, to disguise their age.
• verb transitive To apply a countermark to.