I'm not positive that the STM page you have is quite what you're looking for. I do know this, you will need a high tensile nut because it's going to be under a lot of pressure. I'm all for finding the cheaper alternative to the OEM, but only if it doesn't sacrifice longevity or reliability. If I were you, I'd call up my local Mitsu dealer and get it through them. Can't be too bad, its just a nut, right?
Big 16G, FMIC, Greddy BOV