This is an important tool for striking coins – and very rare indeed. It is an upper coin die for a Roman denarius.
Coins have an obverse and a reverse and thus need two dies. The obverse is struck with the fixed anvil die whilst the reverse is struck with the movable upper die. This die dates from 43 BC, the time of the Roman Republic. The moneyer using it was P. Accoleius Lariscolus.