M5 x .8 Thumb Nut at Cool Screws.
 
One Nut.  The Greatest Thumb Nut in History.  This Beast is 1-3/8 inch in Diameter and Weighs over 3 ounces.  We developed this 5 mm Nut to beef up the Positioning Arm and its unbreakable.  We take every opportunity to make improvements.  This nut is chunky, with a beautiful finish, and perfect knurl, and looks really cool.  This nut is going to work its way into other products but it works so nicely on the Positioning Arm I thought to share it here.