If you added one more digit to your serial number, it would have been 1980 - 1983. Murphydog is correct. Serial numbers N1 - N99999 were made from 1969 - 1972. With a bit of extrapolation (which really means nothing) most likely a 1971 or 1972 date. The N series serial numbers were also used on Models 25, 27, 29, 57, 58, 520 and 629, so they are not unique to the Model 28.