That kit's made by someone called "Archmill". LOL
Made it on my lathe a number of years ago and have sold quite a few on Fleabay. I have never put them on this site as when I joined, the site made it clear that it was not a site for selling manufactured items. Bad manners seemingly.
Anyway, the threaded bar (12mm), locknuts , washers and screws are stainless. Top hats and join bars mild steel.
This kit takes a lot longer to install/dismantle than the Gordon kit but how many times a year do you want to remove the rocker box/head or barrels.