The K-22 square butt 4" barrel 5 screw revolvers started using the model 18 designation in 1957.

The various engineering changes were:
18-1 1959, extractor rod thread direction
18-2 1961, cylinder stop change, deleted trigger guard screw
18-3 1967, rear sight leaf screw change
1968, deleted diamond grips
18-4 1977, gas ring from yoke to cylinder
1982, eliminate pined barrel
1985, model 18 discontinued

They are all great guns and I prefer them over the new ones.