Is yours missing or just not upto standard? You don’t actually see the hex on the adjuster as it’s covered with a sleeve, only the nut is visible. It’s not a difficult item to make on the lathe but I don’t have a 1.25 die, only the more normal 1.5. A local engineering shop might help you - I can supply dimensions. I would go stainless if I was making one but make sure you put copper slip or similar where it goes into the alloy cover.
Ian