I am not an expert on Smiths, so with that in mind....

The Model 28-2 started in 1961, with S serial #'s. In 1969, they changed to the N serial #, which is what you have. They sold 28-2's until 1982, when Smith changed eliminated the pinned and recessed feature and called it the 28-3. While I don't know the numbers they made, it would seem that something made for 13 years would be less than rare. The SCSW shows that there are some variants that may be of interest, but yours seems to be a standard 28-2 as far as I can see.

Others will chime in I am sure.