This was an N frame S&W with a smooth bored barrel, I believe four inches.
At one time there was a rumour to the effect that the shortage of Model 29's in the 70's was due to S&W production of N frames being devoted to the tunnel gun. However, there were never any where near enough tunnel guns to account for any shortage of Model 29's. The tunnel gun was an experiment that never came to much.
The shot cartridge did have a piston to keep noise down. Contrary to what we are used to seeing in the movies, conventional suppressors do not do much to for a revolver because of the sound produced at the cylinder gap.