According to Christopher Frayling Frank (Fonda) uses a reproduction Silver Colt .38 and Harmonica (Bronson) a genuine Colt Peacemaker with the frontsight removed and the hammer smoothed down to improve speed on the draw.
Harmonica's Peacemaker has the front sights removed. As for the dropped hammer, I don't see it. The hammer is not dropped for if it were then the top would be flat not curved up.