VFG Adaptor Swiss Rods - (M5 x 0.8 pitch) female thread.
It is designed to accommodate two felt cleaning elements. The first felt cleaner is screwed onto the thread and thus held fast on the adaptor. By working the cleaning rod to and fro, this can be used to polish, oil, degrease etc. the bore.
The second felt cleaner should only be pushed on, so that in soiled condition it is pulled off at the muzzle when the cleaning rod is drawn back into the barrel.
The knurled nut is only ever used with large diameter felts such as 12 gauge to stop felts pulling off the adaptor inside the barrel.