Again, Colt stamps double action revolver serial numbers in three places:
On the frame under the barrel, on the cylinder crane, and inside the side plate.
904879 was made in 1954, which was the second year of production.
The numbers that year started at 903251, and ended at 908800.
The number scribed under the grips could be anything, probably added by an owner for ID purposes, although why the serial number wasn't enough, who knows?
It's not unusual to find strange letters and numbers on firearms.
Your Trooper is an early model, so it should have early features:
The blue job in those days was Colt's early "two-tone" blue, in which the "edges" of the frame and the flutes in the cylinder were bead blasted to a flat black finish, and the flats of the frame and the outer diameter of the cylinder were bright blue.
The muzzle was left un-blued "in the white".
The rear sight was Colt's early version of the Accro sight.
On the early .38 Special Trooper the firing pin is mounted on the hammer. In the 1960's, the firing pin was moved to the frame.