Here goes, (with some model number examples):
Revolvers:
I & J= small frame revolvers (31, 36, 40)
K= medium frame revolvers (19, 17)
L= slightly larger medium frame revolvers (586, 686)
N= large frame revolvers (27,29,629)
X= giganto frame revolvers (500)
Pistols:
1st generation= original S&W auto loaders (39, 59)
2nd generation= next generation auto loaders (559, 659)
3rd generation= next generation basic design auto loaders (909, 410, 5903, 5906, 4506)
TSW=Tactical Smith & Wesson= newest design metal frame tactial pistols (5906TSW, 4006TSW)
That's right off the top of the head, so if I mis-characterized something, I'm sure that another forum member will correct me.
Welcome and good luck!