In addition to the engineering changes listed above...
29-3 - Dropping of the pinned & recessed features in 1982.
29-4 - Change in the cylinder yoke retention system and the installation of integral scope mounts on the 8 3/8" guns in 1988.
29-5 - Changes to the cylinder stop notch and bolt block in 1990.
29-6 - Change in standard grips to Hogues, all guns drilled & tapped for scope mounting (under the rear sight), and a change in the shape of the extractor star in 1994.
That's the last change my book has.
Unfluted cylinder has two benefits. It's somewhat stronger, and it's easier to engrave with pretty scenes.
As for the full underlug barrel, a number of Model 29 variants have come with that type of barrel, mostly special order or commemorative guns.
The one that springs immediately to mind is one of the series entitled "12 Revolvers," and individually titled "The Attack."
Not all that many were made, as the series didn't garner the kind of interest that S&W was hoping for.
I disagree with Don on one thing..
"leave it grooved."
With a couple of exceptions, all of my S&W revolvers have had the trigger grooves ground down.
S&W for some ungodly reason puts freaking knife edges on their grooved triggers. A short shooting session with magnum level ammo can leave your finger literally sliced open in places.
The alternative is to fill the grooves with some type of epoxy material that can be removed.
Shoot it first, to see if you like the grooves. If you don't, though, don't hesitate to grind them off.