Nilfisk and Tellus Industrial Vacuum Cleaner 50mm Wand Plastic Friction Ring
Nilfisk GM82 and the Nilfisk GS83 and GM83 and the Nilfisk GB833 industrial vacuum cleaner 50mm plastic screw thread friction ring. It fits on the steel wand and a separate metal part screws over it and locks them together. It's under the knurled silver locking ring on the top of the steel tube in the foreground of the photo.
At the moment, standard postage is capped at a maximum of $20 within Australia, so maximise the saving on postage and buy something in the related parts below!