I got an answer from Tiesda at Starboard HQ on your FS screws for the SUP
The inserts are the same as on the windsurf boards, so 6mm x 28 mm PT screws
should fit very nicely!
As far as your X-186, can you send me a photo of the FS inserts in your board
(with one end of a strap removed)?
I may have some of the older FS anti twist plates, but I don't remember the exact
configuration that the X-186 was built with.
Hope this helps,