Audubon Great Horned Owl 1971 Eaton Circular Jigsaw Puzzle Missing 4 Pieces

National Audubon Society's "Great Horned Owl" circular jigsaw puzzle. Over 500 pieces, circular 20 1/4" diameter shape. Appears that about 4 pieces are missing, see pic. This is the most challenging puzzle I've worked on. Box is in great shape with no writing or split corners. Puzzle has been put together and appears to be missing 4 pieces or so (See pic).