Is that a silencer?

Yes.

On a revolver.

Yes!

As I understand it, it can work but generally won’t.

You are three for three.

This revolver is from the 1966 Tunnel Exploration Kit. It is a .38 Smith & Wesson, with a supposed reduced cylinder gap and a silencer.

The final report on the kit was that 3 of the 4 units using it found that the silencer gave no appreciable advantage. Report here.