This variant of the Smith & Wesson Military and Police revolver was known as the Victory model. Made for military use, it can be picked out by the lanyard loop on the butt, and most had a serial number beginning with the letter V. They were made in caliber .38 S&W, and later in .38 Special (you folks called it the .38/200) It was not until well after WW 2 that S&W started numbering their models- but when they did, that would become a model 10.
Only a few oddball European revolvers HAD a safety- they do not need one- the safety lies in the large movement of the trigger needed to fire one. So please do not write about the antagonist "Clicking the safety off on his revolver..."
If you were cocking a revolver, finger OFF the trigger, and hammer slipped, not likely to discharge (anything is possible- I mean, hell, Diana married Charles...)
BUT- if you had a gun that had been cocked, and you were trying to lower the hammer, if you held hammer with thumb, pulled trigger to release, were easing hammer down to rest position when it slipped out from under a thumb (covered in blood or the grease from a Yorkshire pudding)- you get a UB.
Many folks speak of an AD- Accidental Discharge. Here on this forum we use ND- NEGLIGENT DISCHARGE. Also called being stupid in public.
A broken automatic pistol that fires when the safety is released (with no trigger pull) might properly be an AD. Putting your finger inside the trigger guard before you are ready to shoot (and sneezing) was negligent.
Your character may have chosen a British revolver, such as the Webley.
Like the S&W, some of the Webleys shot the same cartridge, a nasty little .38 with a very big bullet. Due to the very long bullet, had a habit of turning sideways when it hit you, making a rather impressive hole.