Last Supper Wall Plaque - old

it is hard for me to determine some things about this plaque. i do not believe it is made of wood it seems almost like clay. It has some cracking and i do not want to try to clean it. It has a raised boarder which looks different from replicas i have seen of it online. The only marking on it is a Gold or bronze color stamp which starts with either a P, B,or R. this makes it hard for me to look up. but as yet i have found none with this mark on back

Fair