What yea did Forbes Silver Co use this mark, please?

I have a tea set that is from the Forbes Silver co, but the mark reads F.S. Co USA . It is in a circle with the forbes eagle head in the center. Their is a capital letter C above the circle . All my research tells me this is Forbes silver, but I can’t find any match to the exact marking. The teapot is #1511. Thanks again for everyone’s help.