It's the revision or version of the model.
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
If you PM me the S/N I can tell you the year the gun was built.