Sadly, it is not a suppressor. NRA Museums posted this on Facebook.

The hollow club extension on this double-action revolver allowed rounds in the cylinder to be fired and then an immediate shift to use of the club if necessary. Intended for law enforcement, the awkward combination of the club extension fitted to the Smith & Wesson Corp. revolver’s barrel did not see extensive use by any agency.

Im not sure how sturdy it would be to use the billy club for hitting people, while attached to the revolver.