That might be an EOC 8 collet
I'm working with a 3-axis mill at the university, and a few collets are missing, but I haven't been able to identify the collet type so far with the charts I've found.
The nut measures, externally, the same as a ER16M nut that I have except for being a tad taller, but the thread is different (smaller diameter) and the ER16 collets are too big for the nut.
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That might be an EOC 8 collet
Jim Dawson
Sandy, Oregon, USA