There's an 11mm "embossed" part of that 21mm "assembly" - it's a little hard to explain, so I'll just quote the technical manual, "Keeping the main shaft nut tight against the shift rod retainer nut, screw the shift rod retainer nut onthe the drive side axle and tighten with an 11mm socket and torque wrench to 20Nm (15 ft. lbs.)"

Yeah, I found out about the new NuVinci just moments after I pulled the trigger on this older model. I'm not really in any position to buy another one, after just shelling out massive Benjamins on this Internally Geared Brick. I guess I could sell this, and take a loss - then try that newer version, but that seems like a lot of hoops to jump through.

Here's the excerpt from their manual in .jpg form for your viewing pleasure: (Step 5 - "d" shows the 11mm part)