The Assay Commission was abolished in 1980; a time when the US was no longer minting circulating silver and hadn't started to mint bullion (aside from the American Arts Medals, which although they were gold, carried no denomination). Now that the mint is producing a range of silver, gold, and platinum bullion coinage, is it time to bring back the Assay Commission?
I think they made other provisions regarding a check on the coins being minted when the law permitting the minting of bullion coins was written. And I believe it is overseen by Congress with reports of assay tests on the bullion being used included in the Mint's Annual Report to Congress. As you can see here in the US Code, there are times when the SecTreas is required to assay coins and bullion - http://www.law.cornell.edu/uscode/text/31/5121
Have you looked at any of the records of the assay commission. They make some of the stupid current political practices look like child's play.