Ah, ok. That then has given me food for thought!
If you look at the arm base in the photo above you can just about see that the base is a direct clone of the Linn collar. It has the second inner radius to form rails for stable pillar contact, almost exactly the same dimensions, and it mounts with three M5 bolts on a 24mm radius at 120⁰.
It also has some features that the Linn version lacks.
Most importantly it has three extra M5 threaded mounting holes which, in conjunction with one of the Linn mount holes, form a rectangle of bolts which can hold a printed clamp under an SME aperture in a plinth. The sliding range is less than the normal range.
Secondly, the underside of the base has narrow ribs to ensure stiff but small area contact with the arm board.
The vertical depth of the tunnel is slightly larger.
The pillar locking bolt is M6 flat nose and has no sprung brake to semi-disengage the pillar. I like that feature but decided that I would have a think about whether there might be a better way to do it in the future, and setting up the Mk 1 Blackbird pillar could afford to be a more edge-of-the-seat experience. One thing I'm not sure about is the size of the contact between the Linn retaining bolt and the pillar. Due to the braking mechanism the contact area is quite small, perhaps less than 4mm^2. I have not done experiments on whether the contact area affects sound, but we all know that contact pressure does, and this small area may be the reason why. After all, metals are elastic.