I found and bought a similar knurl tool at an auction back in the 196os, it has only one wheel and is intended for straight knurls; I have used it only once, back in the 1970s, to knurl a 3/8" dia. CRS rod that was used in a printing press to drive a roll of paper; I remember that it was fairly critical as to center height, as if not exactly on center it will create a spiral knurl of long lead, which for this job did not matter, may have even helped; I made a bushed flat bar to fit my follow rest in place of one of it's regular jaws that ran just ahead of the knurl tool to hold the rod in position to be knurled, I think the rod as finished was about 3 ft long.