You are trying, I'll give you that!
What you need are new "Barrel nuts" for your fin.
Send your address to my email, and I will send you a couple of new
brass barrel nuts for that fin.
Best if you remove the existing nuts (take a flat round punch slightly smaller than the outside diameter of the barrel nut and drive it out of the fin with a hammer) and then measure the outside diameter of the barrel nut (with a micrometer if you have one availavle), and send me that size.
There are lot's of different sizes of barrel nuts.
As an alternative, retap to 1/4"-20 UNC as suggested above.
If that doesn't work, get a Permatex/Loctite Stripped Thread Repair kit and "rebuild" the threads to your existing 6 mm fin screws.
I'm on the road at the moment, but I can give you the kit number and all pertinent details a bit later this evening.
You could email Joey <email@example.com> or <firstname.lastname@example.org> and ask for some barrel nuts, but without the size (outside diameter) of your existing nuts, it would be difficult to send you the correct size for a fin and board as old as yours.
Hope this helps,