There are six (6) or seven (7) notches cut into the grip, not four (4).
It's a Belgian copy, somewhat inaccurately engineered, of the Colt's Model 1877 "Lightning," double-action (DA) revolver, but with some visual features of the Colt's "Frontier" Model or Model 1873 Single-Action Army.
For all I know, its engineering might be an improvement over that of Colt's Model 1877, since the Colt's "Lightning" had a reputation for fragility.
Anyway, it's trying to trade on a superficial resemblance to several different Colt's products.
It has Belgian proof marks, I believe from the Liege Proof House, on its cylinder.
If every important piece of it bears the number "2," then that's its serial number.
That low serial number might make it somewhat valuable, but only if you can figure out exactly who made it.
(There are books about the Belgian arms-makers and their products. It may be worth your research time.)
In its day, it was a cheap gun—maybe even junk. It may have misfired, way back in the '20s, during the fight in which its owner was killed.
But it has historical value, if you can prove its provenance in a satisfactory manner.
It's a real, old-west artifact.
A signed statement from your grandfather, or from its previous owner, would be very useful, as would the name and hometown (or home state) of the dead Mexican cowboy.
Whatever you do, do not try to fire it!