I am trying to chase up this nut locally, I can get it out of the US from the supplier but the per piece price is expensive(also need to factor in the delay and freight) so I'd rather buy it locally. My local fasteners shop haven't seen it before/didn't really have a idea on what it was called. They said it looked similar to a Sex Nut however mine is counter sunk and is also open on both ends for the bolt to pass through. It is a M10 thread. To me it is like a barrel nut.
https://ibb.co/1LN2TSK
https://ibb.co/gyk83hN
https://ibb.co/Yhjhvns
Here is a screen shot from the suppliers website, they call it a "PAPILLON NUT"
https://ibb.co/w44YsmR
Not a clue but a marine shop like Whitworths might just have it. They tend to stock tons of unique parts for American boats and trailers etc that you won't find at Bunnings.