Swage

[Luis Fok]

A tool, variously shaped or grooved on the end or face, used
by blacksmiths and other workers in metals, for shaping their work,
whether sheet metal or forging, by holding the swage upon the work, or
the work upon the swage, and striking with a sledge.

[Gregory]

A shaped tool or die for giving a desired form to metal by hammering or pressure.