I do agree with cogar In very very old homes plaster was used to make very ornate design accents such as ceiling medalions. Now they make some of high density urethane as they are cheaper & more durable than wood or plaster. I would take it to someone who specializes in such accents. or send photos to a local auction house to see if it sparks any interst. As far as the marking if held as it was meant to be hung it looks like a cross on the peak of a roof (just guessing on that one) It may have some religious qualities the look just draws me that way. Good luck
