I have two of these on my aft cabin door, aft side. (H376). But on mine, since the day I bought her in 1996, there has also hung a "Grab Bag". This is a canvas contraption about a yard long with about four pockets across it each having a velcro flap to retain the contents. Each end is the canvas strap handle which fits over the moon shaped wooden item shown in your photo. I also have two of them, the lower one with its straight edge uppermost to receive the lower handle and keep the whole thing from swinging about.
Though I have never used it, I regard it as a neat afterthought to the boat's design.