Just tidying up some blasts from the past (including some very ancient ones), largely for the benefit of anyone searching the site for information.
That’s one of the marks used on silverplate by George Waterhouse & Co. When you see an ‘S’ on its own after, but not part of, the maker name (and often in a different-shaped cartouche), it almost always means ‘Sheffield’, England and that’s the case here.
The company first appears in the Sheffield register under that name in 1842 at 75 Carver Street and was active until at least the 1930s. I would think the fork is probably late Victorian or early 20th Century.