The two knurls are on opposite sides of the work so that the pressure of knurling is equalized.
It is actually a knurling process, you can see the knurling marks.
Concave knurls are sometimes used for knurling rounded edges on screw heads, etc.
Another machine puts the knurling around—forming the lubricating grooves, and another groove.
"hard excrescence," c.1600, probably a diminutive of Middle English knor "knot" (c.1400), related to gnarl, from Proto-Germanic *knur- (cf. German knorren "a knotty excrescence"). Related: Knurly.