What is a silver trade unit 1981 fomely stored at san fransico with a mint mark of two c's overlapped and words usassay office one side trade unit on the other dated 1981 Carson City mabe?

This is a privately issued piece and is called a 'silver round', usually a one Troy ounce (31.1 grams) of silver. It should have .999 and the weight somewhere on the piece.