I was looking for information on this bar. I do not see a way to post a photo of it in this forum.
Here's some information about the company. This is a pretty definitive, and makes no mention of assay bars. Beware of "fantasy" issues from this and other pioneer / gold rush periods.
https://www.rfrajola.com/MayerGold/MayerGold.htm#J.J. Conway & Co., Bankers
I’d love to see a picture. I know the person/company who may be able to authenticate it.