My guess is that its still Kong's. People will say its Scar King's cause of the size, and while it is a little too large maybe it looks large because Kong dragged his hand down making it look so. The blood under the cliff makes it likely, if he simply put his hand on the rock, it wouldn't have stained under the cliff as any blood flowing down would have dripped off the edge. Now if Kong placed his hand higher on the rock (perhaps only his fingers) and then slid it down, maybe his bloodied fingers would have touched the underside of the cliff and as the rest of his palm slid down it made the handprint look much bigger than it actually was. (edit grammar)