40MM round Delrin Acetal black plastic.

These are off cuts from the machining process. An off cut may feature machining marks at one end and a chamfer at the other. 

The length listed is for the unmodified bar. For example, a billet listed as 100mm may measure 105mm, but has a 5mm chamfer at one end.

Please note that while the bar is sold as 40MM diameter but generally is about 40.5MM in size.