EWTN Catholic Q&A
Silver Hammer
Question from Rosalind on 04-03-2005:

Why is the pope's forehead struck with the hammer? When did that practice begin?

Answer by Matthew Bunson on 04-08-2005:

The practice is an old one, although its precise origins are shrouded in mystery. There is some debate as to when it was last performed, with some experts saying 1903 and other 1958. We know that it was not conducted at the passing of Pope John Paul II.