Facts and Celluloid 
In preparation for the reveal that Arch Stanton’s grave was not the one containing the coveted treasure, Set Decorator Carlo Leva had a hard time producing a prop skeleton that met Sergio Leone’s standards. Convinced the counterfeit would not sell on screen, Leone directed Leva to try and find authentic human remains.
While on this search, Leva heard of a woman in Madrid who had a real skeleton for hire. Upon inquiry he learned it belonged to the woman’s mother, who in life had been an actress. Shortly before passing, she had expressed the wish that her body be used in movies so that she could continue acting even in death. Needless to say, Leva rented the skeleton, and it is her body you see in this scene.