Genuine Herman Miller Eames Aluminum Group Series control knob with bar for the recline tilt mechanism.
Thank you for looking and don't forget to check out my other Eames items listed:
https://www.ebay.com/sch/i.html?item=194696096249&rt=nc&_trksid=p4429486.m3561.l170197&_ssn=midnitecrasher