On my Ruger that button only releases the cylinder for reloading.Actually UK police only used S & W revolvers back in my day.
This S & W .44 I'm using here in the late 80s had a safety catch.
Forgot to say that of course MOST revolvers do not have safety catches, but the ones I've encountered that do have them, have mostly been military and police issue, which would cover CIA as well.
I've seen 2 types of safety catch on revolvers, one which converts the chamber release on the left side into a safety catch and one which is up behind the chamber below the hammer, which is a simple psuh/slide bar to lock the hammer.
An officer at my old shooting club had a Ruger with this type of safety catch (that said he also had a Desert Eagle, a very ornate Colt .45, and various other automatics and revolvers, including a WWII Russian automatic)
I know some of the old British Webleys and BSA revolvers had safety catches as did some of the early German revolvers. The first police revovler I handled in London in the early 80s was the S & W Model 10 (5-chamber) .38 Special and that DID NOT have a safety catch.
2009 - That's the butt of a Swedish Army Pistol 88 (9 mm Glock 17) you can see in my breast pocket...and yes...I am a leftie.