its one of the grub screws in the fins ...
Aaah, sorry, I did not get the "topic headline" which said it was the fin screws loud and clear - my mistake...
So it is the small "Allen key" fin screws.
Still, a small screw extractor might do it.
To prevent the hex from damage, one has to be careful, as there are two almost identical screws, but the Allen keys dont match
One is the most used Future and FCS grub screws which are typical US/Imperial, whereas Naish uses a Metric size for their FCS/DFS fins.
So one is 2.38mm (3/32"), the other is 2.5mm.
And they dont fit into each other - and such a small difference that you can almost not measure it
We have often bothered A LOT with this, when on wave trips together, and "borrowing" Allen keys from each other
I've also seen some "cheap" fin keys, that are not 3/32 nor 2.5 and looks like something not belonging to any standard
They also tend to be damaged really easy - too soft and not hardened material, so they get messed up fast