It's the M5 plug-in nut shown as p/n 46637651465. I also have a spare one sitting somewhere underneath the ECU box.
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Present - 2016 340i, Alpine White, Black, M-sport, 6-spd, Track Handling, Tech pkgs
Past - 2011 E90 335i, Alpine White, Ivory, Sport, Premium, Steptronic
Past - 2008 E90 335i, Titanium Silver, Dakota Grey, ZSP, Steptronic, 6FL, Wagner FMIC, Bilstein B12 Pro-kit, M3 control arms, SS F/R brake lines