I had a 19-4 overstamped to a 19-5.

The 19-3 was in production when the 66 was introduced and so is the revisional equivalent of the 66 (no dash).
The 19-4 / 66-1 revision relocated the gas ring from the yoke to the cylinder,
The 19-5 / 66-2 revision introduced the non recessed cylinder to the KT357 as well as deleting the barrel set pin.

Transitional models pop up now and again with features from both revision but my guess is the frames were already serial numbered and model stamped when S&W changed to non recessed 357 cylinders,
Instead of making more recessed cylinders or tossing the frames they simply over stamped the model revision and used the new non recessed cylinders .