Hello Simon,
I am not certain, but I believe that one vent forward of the exhaust outlet and above the row of vents may be for a foot / hand hold intended for boarding the boat at this location. Some photos and drawing that include the harbour railings show two closely spaced stanchions on the deck at this point in the hull. Also, somewhere I remember this being mentioned as a boarding station - sorry can't recall the reference. The David Westwood book has a good diagram showing this on page 26/27. I hope this helps you!
Cheers,
Bill