The last serial number beginning with the letter S is S333454 and was shipped in late 1969. The N prefix serial numbers followed and I believe the first production run using the new series was a run of Model 28-2s in early 1970 (N1 is a 28-2 as is N925...the latter was shipped in May 1970).
N3 is a 29-2, N25 is a Model 25-2, and N27 is a 27-2, but these numbers were reserved for these particular guns and made at different times. I don't know if N29 is a Model 29-2, but it would be neat is if it is.
Bill