As with all the current production Smith & Wesson revolvers the firing pin is internal and not part of the 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 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.
Since the hammer nose
of the Sentinel is the firing pin, to prevent the hammer nose
from striking the rear of the cylinder and possibly fracturing if dry fired, there is a relief groove milled at the 12 o'clock position on each chamber.
Pushing down on the button raises the transfer lever so that it's directly behind the firing pin and can be struck a blow by the hammer nose