It's not a physical mark.
Those who belong to God have His mark on their forehead. It represents allegiance. The hand represents actions/authority
Vicarius Filii Dei (Latin: Vicar or Representative of the Son of God) is a phrase first used in the forged medieval Donation of Constantine to refer to Saint Peter, who is regarded as the first Pope by the Catholic Church. The pope's title “Vicar of the Son of God” in Latin is Vicarius Filii Dei. When you add up the Roman numerals in this title, you get 666.
They claim Mary is Jesus mother but she only birthed the temple, the body. He made it clear who His mother was. They claim Mary sits at His right hand, exactly what Solomon did, who built the first temple and set a throne at his right hand for his mother. Jesus said this position wasn't His to grant. Solomon received 666 talents of gold yearly, not including revenue. Mary is symbolic for them.