Our family has a Colt Single Action Army, serial number places it in 1974, with two notches on the left grip next to the grip frame.
This gun is known to have killed two burglars in a logging camp near Eatonville, Washington state, about 1920. Perhaps a decade earlier.
The gun was given to my great uncle as payment for $20, which the burglar killer did not have.
My father was given the Colt about 1930, when he was 15, and it had two notches on it then.
Now, whether these were applied for the burglars, or an earlier killing, remains unknown.
From what I have read over the years, and I believe that the late Elmer Keith wrote this long ago, putting notches on pistols was not common but not unknown.
The writer -- remember, I can't recall who -- said that when applied, notches were typically placed on the grip next to the body. They weren't as visible this way. Carved into the outside grip panel, they were too noticeable and might spur challenges.
I don't know how far back the practice goes. I doubt it was done much in the 19th century. Keith did write that gunfighters didn't get into fisticuffs, because they couldn't afford to injure a hand.
I also rather doubt they would have provoked a gunfight with someone looking to make a reputation by such a blatant display on their revolver.
But through the ages, there have been soldiers who cut off body parts of their enemies as souvenirs, and to show a personal body count. One can't overlook the fact that such personalities might have carved notches on their gun, way back when.
"And lo, did I see an ugly cat. Smoke. Brimstone. Holes in parchment. And this ugly cat was much amused." --- The Prophesies of Gatodamus (1503 - 1566)