I bought some threaded rods with hex centers from my favorite nut and bolt supplier who deals with new and surplus material.
At the time I did not have a plan for them. They were metric threads but I thought I could use the stock at a later date.
So far I built 4 machinist jacks from them and now I added these MT-0 and MT-future taper adapters.
You can see in the pictures two that have been completed and 4 are still blank for future use.