You are buying a of pair knobs, one top and one bottom.

Dual control metal barrel control knobs for use with dual pots. Great quality with knurled edges and tightening grub screws.

Top barrel is 12.5mm high / 19mm diameter.
Bottom barrel is 10mm high / 25.2mm diameter.

The top knob has a 6mm diameter hole.
The bottom ring has a 8mm diameter hole.

Great for replacement or upgrade.