There was no notch on top of the revolver, but there was a notch found on the hammer nose
A dual firing pin system could be switched to position the hammer nose
over either the rimfire or centerfire firing pin in the frame.
The 1858 Remington has deep notches for the hammer nose
and is the safest of cap-and-ball revolvers.
The plate that was soldered in the slot is now cut to fit flush with the top of the frame and slotted in the center to a few thousandths wider than the hammer nose
The sixgun, as it originally (and most popularly) appeared, had an open top, a la Colt, and a rear sight notch in the hammer nose
that could be lined up with the bead-style front sight when the gun was on full cock--also similar to that of the Colt.
This can be accomplished by crossing the shooting hand thumb behind the hammer and shoving down on a stud on the moveable hammer nose
The front sight is a little gold-colored post with a rounded top, and the rear is a pair of rounded channels in the top of the frame, with the slot for the hammer nose
at their lowest point.
The firing pin or hammer nose
penetrated the open space at the rear of the chamber to dent the "cup primer" at the rear of the cartridge.