From http://www.handloads.com/misc/Smith.Model.Changes.asp
Model and Dash numbering system
--------------------------------
Model 10
-1 1959 Heavy Barrel
-2 1961 Changed extractor rod thread to LH on standard barrel
-3 1961 Same as above, for heavy barrel
-4 1962 Screw in front of trigger eliminated
-5 1962 1/10" to 1/8" front sight, on standard barrel
-5 1962 Screw in front of trigger guard eliminated on
heavy barrel model
-7 1977 Change to put gas ring from yoke to cylinder
-8 1977 Change to put gas ring from yoke to cylinder
on heavy barrel model