The cutter is of course rotating, but yes the X axis is the only one feeding. It would have a farily high feed rate. What ever the space between two adjacent arcs is then it was feeding that much per revolution per flute on the cutter. If you lay a straight edge square across the rib & compare where the arcs end on each side of the rib it is quickly apparent it was cut with a moving cutter. If the cutter had been brought down to depth & cut an arc, then raised & the rib moved to a new location for another cut etc then the run off on each side would have been on a line square to the rib, but they aren't. You can even determine whether the mil was traveling from breech to muzzle or muzzle to breech by the cutter runout.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra