Hawksmoor's St George in the East

Chalking the doors at Epiphany

"The first and last numbers simply refer to the current year. The letters C M B come from the traditional names for the three kings: Caspar, Melchior, and Balthazar. They also stand for the Latin sentence “Christus Mansionem
Benedicat” which means, “May Christ bless this dwelling.”
https://learning.leeds.anglican.org/resource/chalking-the-door-an-epiphany-tradition/