Last Supper Wall Plaque - old

this is a plaque of the last supper. I am not sure of the medium. It may be wood but I do not think so. It has a clay like texture. I do not know exactly how old it is. I do know it belonged to my great grandmother, so at least 75 years old. It has a stamped marking on the back which I have taken a picture of but can not read it. Any info would be helpful. thank you.

Fair