nah (that looks like something out of benders toiletry bag)

a fence fin was typically a poly carbonate fin with 2 or 3 'fences, runing around the foil
parallel with the bottom of the board.
they where ridges which where i am guessing around 10 mm high and at 90deg to the foil. they extended forward of the leading edge and aft of the trailing edge aprox 10mm
(guess). they where designed to stop (i think) ventilation travelling down the fin.
they where great for grabbing cray pots and sending ones self in to orbit. funny enough they where also good at grabbing weed and sucking in the sailing consumer of the time.
you can see similar gadgets on air craft wings as well (i think from memory multifins australia sold the rights to boeing for a carton of piss, but i could be wrong on that)

there where also many variations of the theme from what i described above......