N300000 to N399999 was 1975/76.
N400000 to N499999 was 1977/78.
N500000 to N599999 was 1978 to 1980 with most being 28's and 57's.
N600000 to N699999 was 1979/80.
There was overlap depending on individual models.

A 57-2 would be the new yoke retention system (larger diameter spring loaded screw), but that did not start till 1988.

The dash numbers refer to engineering changes, such as the new yoke screw for the 57-2 and the elimination of the pinned barrel and counterbored cylinder at 27-3, 29-3 and 57-1.

A great reference book is the The Standard Catalog of Smith & Wesson which explains most of this and provides tons of other information.

Jim