What is Assay Office?

Legal Definition
Assay offices are institutions set up to assay (test the purity of) precious metals, in order to protect consumers. Upon successful completion of an assay, (i.e. if the metallurgical content is found to be equal or better than that claimed by the maker and it otherwise conforms to the prevailing law) the assay offices typically stamp a hallmark, punze, or poinçon on the item to certify its metallurgical content. Hallmarking first appeared in France, with the Goldsmiths' Statute of 1260 promulgated under Etienne Boileau, Provost of Paris, for King Louis IX.
-- Wikipedia
Legal Definition
The staff of persons by whom (or the building in which) the process of assaying gold and silver, required by government, incidental to maintaining the coinage, is conducted.
-- Black's Law Dictionary