If I recall also Smith dropped the
grooved triggers for wider, smooth
ones, and at the time may have
introduced a "larger" cylinder
retention screw.

And the tapered barrels such as
on the Model 15 were dropped for
the straight "heavy" barrel.

I think of the cutoff date being 1981 with
the changes coming in 1982.

Then internals were changed somewhere
along the 1990s such as the armature
for the hammer spring, it no longer being
pinned but free to fall out.