I'm reading the Dame Shirley letters, and many times she mentions having oyster dinners, oyster soup, and so on. She also mentions how difficult and expensive it was to bring supplies up to Rich Bar. Would these have been tinned oysters, or would the restaurants have made an especial effort to bring fresh oysters up from the bay area?
They would have likely been fresh oysters - and they would have been Pacific Coast oysters which have a smaller, rounder shell than Atlantic counterparts. It wasn't easy, but freshly harvested oysters were rushed to the hinterland regularly. I wrote about this in my book Virginia City: Secrets of a Western Past (2012).
The sudden presence of Chesapeake Bay shells (larger and elongated) is a clear indication that one is in a post 1869 archaeological site: the transcontinental RR allowed for the importation of superior East Coast oysters to the West Coast.