The skull and crossbones at the feet of Christ wasn't put on the cross after 1835 is my understanding! I own a cross much like that one. Hope this helps.
And this much less interesting explaination: "You will see on some Crucifixes a skull and crossbones at the foot of the Cross. Aside from symbolizing victory over death, this skull more specifically represents the skull of Adam, said in Jewish and Christian tradition to have been buried at Golgotha, where Jesus was crucified. The Blood of Christ, the New Adam, redeems man, as symbolized by the skull of the First Adam. I Corinthians 15:22, 45: "And as in Adam all die, so also in Christ all shall be made alive...The first man Adam was made into a living soul; the last Adam into a quickening spirit."
Google "crucifix with skull crossbones"... there is a lot out there