If I were you,, I would remove it from the frame and see if there is publisher info either on back or in the border !! Poster board is not something he would have used as far as I know !! Just remove the clips from the back and lift the backer board out !! Sometimes the artwork comes out with it,,lift it gently by the edge !! Wear gloves so the oil from your hands does not soil the paper. !! Put it back the same way it came out !! Its really simple !! I would clean the glass well and let dry before putting it back in the frame !! Just handle it gently !! Just in case !!