Hey Bill, hey all,
When @ben mentioned the Mafell FM1000, one of the numerous machines with 43 mm euro mount, and the need for a 65-mm-to-43-mm reducer, I immediately had the thought that I would really like to try to make a reduction clamp for 43 mm tools for the Onefinity mount. I don’t have the time at the moment, but I think with some experience, one should be able to mill one out of aluminium. I imagine it would clamp the 43 mm machine with a small (<1 mm) radial opening on one side, tightened with one or two grub screws, and it would look like a 65 mm or 80 mm diameter disc. But the problem is, the Onefinity mounts have certain height (1.5″ (38.2 mm) height on 65 mm mount and 2″ (51 mm) on 80 mm mount), and since 43 mm Euro Mount is not that deep, the milling motor would either need to be slipped into the mount but at least the Suhner is to wide for this (see image below), or the collet would not protrude completely out of the mount on the underside. Or, third possibility, one would take a file and file down the Onefinity mount 
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UAD 25-RF cross section in Suhner Catalog