I have attached a paper from Renishaw which mentions this with the key word 'trilobular'.
It is also mentioned in US Patent 5,146,691 awarded to Renishaw.
You might find this paper of interest as well:
https://www.precisionballs.com/Micro...Components.php
and this one too:
http://www.measurement.sk/2010/S1/Salah.pdf

It is commonly said that down in the ultra range, halving the error costs 10x more.

Cheers
Roger