The "N" prefix serial numbers began in 1969.
The model 28 was made from 1954 to 1986. There were 3 engineering changes to the model 28, yours would be the second engineering change.
The second engineering change happened in 1961. The third engineering change happened in 1982.
Yours was made sometime between 1969 and 1982, that's the best I can do at this time.