Countermarked

(en) A countermark is a mark (letters, numbers, or symbol) added by punching with a hammer onto an existing coin (the host coin). Since antiquity, countermarking has been used, for example, to indicate that a previously demonetized coin was once again considered legal tender; this technique has also been used to signify that the original value of a circulating coin has been altered later on, or that a foreign coin is considered legal tender. This practice is now obsolete.