Fostex B16 Tension arm cap
This was from the Model B16 but may have been used on later models, in principle on some of the machines there was a steel pin on which was a bottom metal bush that locked onto the pin, then two bearings was slotted through the pin and finally on top there was a cap .
Both the cap and the bush have set screws in them to lock them into place, the whole assembly then slides into the tension arm where the pin is tightened up with a screw that holds it into place.
All these parts are rare now as get broken or lost on machines.
Complete arm sets are cheaper and listed elsewhere.
A customer in Geneva pointed out that they have a 3mm hole he wanted one for a g16 which is bigger so he just drilled it out