Probably fits into a hole at the end of a spinaker pole or similar. Is there a fitting of some sort directly above it on the same side of the mast, maybe 5 to 8 feet distant? Wouldn't be a matching "hook" necessarily, just a fitting of some sort.
If the boat is new to you, look below decks for a spinnaker pole, or a jib pole, etc. If you find one, check the ends.