The Standard Catalog of Smith and Wesson shows the N-prefix serial numbers in the 200000 series ran from 1974 to 1977. Others will be along with better information. My guess would be 1975. As is debated at some length in this thread, the original grips would have been wood, either the Magna (much more common) or the Target style. You can probably find correct original grips, if that matters to you, on ebay or gunbroker. There are also several sources of present day custom grips in various materials. Your 28 looks to be in very good condition. I purchased one about a year ago from a private party for in similar condition for $500, also without original grips.