So smart a$$ to the rescue

It is not a number, but a hook symbol. What is missing is a hole or a small plug covering said hole. As you can well imagine, today's removable bits of aircraft are rather heavy and rather large. To that end, many parts of large aircraft have specific lifting frames to remove said parts. These symbols identify the parts that require help to remove them and the attachment points for the lifting harnesses. A few weeks back I had to remove an inboard flap from a 737-800. Not that heavy, but large enough to cause problems if not lifted off with lifting gear.
Matt