Your gun is a Spanish copy of a S&W.
Look closely at the marking you posted the photo of. It says NORMA 7.5 Nagant. No manufacturer would mark a gun with a specific ammunition manufacturers name! There is also a flat on the barrel where the original caliber designation was stamped before the current caliber marking was hand stamped, and quite clumsily at that.
These Spanish guns were made in several calibers, mostly .38 Special, .32-20, 8mm French Ordonnance Revolver, and .32 S&W Long. Chances are this was either a .32-20 or 8mm French, a long ago owner couldn't get the correct ammunition and shot what he found that would (sort of) fit the gun and remarked the gun for that cartridge. This points to the gun spending its' earlier days in Europe somewhere. The picture below if from your photo.
It probably has not been re-chambered. If you look through the chambers and see one step it is probably the 8mm. If there are two steps it is a .32-20.
There are three Nagant revolver cartridges, Russian, Swedish and Swiss. Both the Swiss and Swedish are designated 7.5mm, the Russian as 7.62.
If there are markings on top of the barrel let's see a photo of them. How it is worded will determine if the gun was Spanish or Belgian. The Spanish guns were generally the better (cosmetically) copies of the S&W, and this looks like many Spanish guns I have seen. It should be obvious, but if the gun was a S&W it would be marked clearly somewhere! You are not the first to overlook this.