I think it's a custom thread. The squared off shoulders are possibly to minimize the potential for stripping the sprockets' brass threads. You might consider switching to the older stainless sprockets if you want to play with mast material. The mast ends are simply cotter-pinned in place.
The greatest stress point on the masts is where they exit the sprockets. You might be able to minimize this tendency by giving your masts a small taper. I don't think this would have any negative effects on the fins.
Let us know what you find!
