In Commonwealth Realms such as Britain, Canada and Australia, defacing coinage is illegal, as the coinage is still regarded as crown property. Here are links to the
Australian law, the
Canadian law and the
British law. Of the three, the British law is the vaguest, speaking only of destruction of coins, rather than defacement, so in Britain it would depend on how badly the coin was damaged by the counterstamping; the 1936 law specifically mentioning defacement was repealed in 1981 under new anti-counterfeiting laws and never formally replaced.
You'll probably find anti-defacement laws in place in most monarchies. In non-monarchies, it's hit-and-miss - some countries have laws prohibiting it, some don't. South Africa, for example, prohibits anyone who...
Quote:
wilfully defaces, soils or damages any note of the Bank, or writes or places any drawing thereon or attaches thereto anything in the nature of an advertisement, or wilfully defaces or damages any coin which is legal tender... liable on conviction to... a fine not exceeding R250;
Of course, the only laws you need to worry about at any given time are the laws of the country you happen to be in at the moment you are considering undertaking such defacement of coins from that country. I'm not aware of any country anywhere with laws against defacing foreign coins. So it's perfectly OK to deface South African coins in Zimbabwe, or Canadian coins in America.
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