I can't be too much help here.
The fork is indeed a 'fiddleback' style with 'finned' shoulders, a stout stem, and a 'tipped' terminal on the handle. The problem is that this style first appeared in America around 1820 and almost every maker used it at some time or another well into the 20th Century. They're almost impossible to date, even if the maker can be identified.
I would agree the 'NS' almost certainly stands for 'Nickel Silver' (ie no actual silver content) which at least puts it after about 1830. For flatware, Nickel Silver was used both unplated and plated. If the yellowish tinge is showing preferentially on the tines of the fork, that's probably an indication that it was electroplated and the plating has begun to wear off. That would put it after about 1840.
I don't recognise 'S&R' as a maker, with or without the additional 'S'. The issue I have is the ampersand being between the 'S' and the 'R'. I'm not aware of any of the Rogers' companies using 'S&R' in that way. I don't think it's Sears & Roebuck either. As far as I know the company never used the abbreviation 'S&R'. Although folks commonly refer to the company as 'Sears & Roebuck', the first combination of 'Sears' and 'Roebuck' together in the company name was as 'Sears, Roebuck & Co' in 1893 and the ampersand never went between the two names thereafter.
The only potential maker/retailer using 'S&R' that I know of was Sayre & Richards of New York but they were active from 1802-1813, which is too early for this fork. There are still dozens of manufacturers who just used their initials as a maker mark from the mid 1800s through to the early 1900s (and even beyond) who remain unidentified (some of them short-lived). I guess this is one of them.
