Assay

Assay refers to the testing of a metal/ore to determine the ingredients and proportional concentration of the metal in question.

Assay Mark - a mark that is present on precious metal items that have been certified by an office and approved for distribution/sale. Furthermore, each region (nation and town) commonly have their own variation of an assay mark which allows one to distinguish the origin of the antique item. 

Example:

Birmingham (Anchor - First on the left).
London (Lion's Head - Second from the left).