Hello and welcome to the forum.
This firm regularly registered punches with other Assay Offices and some of these didn't have the marks under the "S" and the "D". Oddly, as it was a Sheffield company, these minor changes don't seem to have been registered there prior to the assay date of the spoon.
The relatively wide disc suggests the spoon stood in the neck of something at one time. The number of Scottish snuff mulls around indicates they were partial to a pinch of the stuff and the possibility that the highland dancer stood on top of a table snuff pot?