A more thorough explanation:

An assayator regis, also known as an assayer of the king, was an officer of the royal mint who was appointed by St.2 Hen. 6, ch. 12. Their job was to receive and test bullion that was brought in for coining.

When someone brought in gold or silver to be turned into coins, the assayator regis would test the metal to make sure it was pure and of the correct weight. If the metal passed the test, it would be sent to the mint to be turned into coins.

For example, if a merchant brought in a bag of gold nuggets to be turned into coins, the assayator regis would weigh the nuggets and test them to make sure they were pure gold. If they passed the test, the merchant would receive coins of equal value in exchange for the gold.

Another example would be if a nobleman wanted to have a new set of silverware made. They would bring in silver to the assayator regis to be tested and weighed. If the silver passed the test, it would be sent to a silversmith to be turned into the desired items.

These examples illustrate how the assayator regis played an important role in ensuring the quality and purity of the metal used in coins and other items made from precious metals.