Microphone stand spare part:
Plastic boom head tightening nut (M8)
Konig & Meyer 01-83-980-55 is a threaded knob, used to fasten the swivel section of a boom arm.
For something a little more fancy, check out the 'barbell' version: 6-30351-1-55
You'll always find a range of genuine K&M spare parts for stands in stock at Factory Sound.