It's a wooden knob that has a small 'hanger bolt'. The side that you see is threaded as a bolt, the other end (in the knob) is threaded like a screw.

https://www.amazon.com/20-1-1-Hanger-Bolts-pack/dp/B001DT4T7C

The post has an insert nut; it has threads on the outside that are used to hold it to the wood, and smaller threads on the inside to hold the hanger bolt. Like this.

https://www.amazon.com/Hillman-3021-8-16-Inch-Inserts-8-Pack/dp/B00HYM05ES