The Collaborative International Dictionary of English v.0.48:

Countermark \Coun`ter*mark"\ (koun`t[~e]r*m[aum]rk"), v. t.
   To apply a countermark to; as, to countermark silverware; to
   countermark a horse's teeth.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Countermark \Coun"ter*mark`\ (-m?rk`), n.
   1. 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.
      [1913 Webster]

   2. (Far.) An artificial cavity made in the teeth of horses
      that have outgrown their natural mark, to disguise their
      age.
      [1913 Webster]