I got this from handloads.com:
N-Frame Model Blue
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Model 20,21,22,23,24,25 (except 25-5) 26,27,28,29
-1 1960 Change to LH thread
-2 1961 Cylinder stop changed, hole in front of trigger
guard eliminated
-3 1982 Eliminate cylinder counterbore (magnums only)
-4 2004 Thunder Ranch 44 Special, fixed sight
Model 57 1964 Introduced
-1 1982 Eliminate cylinder counterbore
Model 629 1980 Introduced
-1 1982 Eliminate cylinder counterbore
Model 25-5 1978 -5 means 45 Colt caliber
125'th anniversary model, -3 (standard) -4 (delux)
both of these used a shorter than standard cylinder.
-7 1985 45 Colt, 5" barrel, unfluted cylinder
K-Frame Stainless Models