journeyman's piece

Likely a calque of German Gesellenstück. Much use seems to be in reference to Germany or continental Europe.

journeyman's piece (plural journeyman's pieces or journeymen's pieces)

1. A work that someone produces in order to prove that they have completed their apprenticeship in a craft.
2. (figurative) An action or achievement by which someone proves their competency.

• masterpiece, masterstick