You are lucky Hugo as your Jelco 750 tonearm bases are newer production than mine. Please note the picture of mine below and how it has a perfectly round hole for the arm pillar. That is the major change that Ammonite and other aftermarket makers changed to the Jelco 750 base collar. They cut away part of the perfectly round hole with a partial secondary hole of a slightly smaller diameter, offset slightly, so that when the arm pillar locking bolt was tightened it would force the arm pillar against the two sharp corners of the secondary smaller cutout to more firmly hold the arm pillar in place. Jelco incorporated this change themselves in their new models, and also in the late versions of the 750 that you photographed.Hugo wrote: ↑09 Feb 2019 10:53
The collars that Jelco supply with the new TS and TK series arms are actually identical to the old collars, except for colour (the new ones are silver). I’ve just taken a few photos to show the two collars – the silver one is from a TS-550S and the darker one from an old SA-750D (my poor photography makes it look black, but it is grey).
This is the Carbon Fiber armboard I made for the 12 inch 750 on my re-plinthed Lenco L75, and you can see the perfectly round arm pillar hole of the early production 750 base collar: