I have both a model 19-3 and a Model 19-5. According to the "Standard Catalog of Smith & Wesson" (Supica /Nahas) The Model 19-4 moved the gas ring from the yoke to the cylinder. My -3 and -5 gas rings are identical. The counterbored and non counterbored cylinders are appropriate for the model numbers I have. What is the story here?

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SASS 976, NRA Life

At one time I had two 19-3s

Gas rings were not in the same place.

One of them was a Texas Ranger 19-3

The other a nickle 19-3 I still have.

The gas ring change is believed to have occurred around the middle of the 19-3 production run and likely extended to very early 19-4's as S&W used previous version parts until their supply of older parts was exhausted. Placing the gas ring on the yoke required a far more aggressive cut into the barrel extension and this aggressive cut was continued even after the placement of the gas ring was returned to the cylinder.

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VCDL, GOA, NRA

The gas ring was first moved from the cylinder TO the yoke, and then later (~1977) BACK to the cylinder when the first change caused more problems than it solved.

The SCSW lists the approximate revision and year when the second change occurred, but does not mention the first change. Apparently neither change warranted an engineering revision, so there is no definite correspondence to a dash number. As other people mentioned, S&W continued to use parts with the gas ring on the yoke for some unknown time.

The first change is what led to making the flat undercut at the forcing cone bigger, and it stayed that way (making the barrel weaker in that spot) even after the second change. The bigger undercut is blamed for the cracked barrels on the Model 19 and Model 66, when police departments switched to training with more and hotter .357 loads. Articles I read reported barrels cracking after as few as a dozen "hot" loads in testing.

A number of those revolvers were returned to S&W for warranty service. That in turn led to the development of the stronger L frame.

Most of this information is in an article that was linked in a separate thread called "Model 19 strengthening".

Model 19 strengthening

Another interesting bit of trivia is that slots were also machined into the gas ring when it was on the yoke to attempt to direct the hot gas away from more sensitive areas. In other words, S&W already knew that the first change did not solve the problem on its own.

The gas ring on the yoke only lasted a couple of years, could be your M19 has the "1st gen" cylinder ring. My M19-3 (1974) also has the cylinder mounted gas ring, looks just like the one on my M66-1 (1980), so they're on opposite ends of the change from cylinder to yoke and back again. The counterbored/non-counterbored feature has nothing to do with it, that was a different engineering change that happened in 1982 and your dash 5 is the first dash to not have the P&R feature, but is well after the gas ring went back to the cylinder. This article talks about the change, plus a lot more to do with the M19's forcing cone.
The Smith & Wesson L-Frame Story - RevolverGuy.Com