O-D;
Will try & explain what I am saying. The flat patern can be reproduced with a single pointed cutter having a diameter just bigger than rib width. Axis of rotation would be perpendicular to rib surface. feed per revolution would be such as to give the desired spacing between the arcs & also to have the trailing edge track the same cut in center as the leading edge. This creates that median & the outer arcs form the little side diamonds.
On a concave rib to use a cutter such as this it has to be tilted of perpendicular so the leading edge follows the concavity. The trailing edge then would not touch at all thus the cross cut is not made. Is hard to say positively from the picture, but appears perhaps a smaller dia cutter may have been tipped & several passes made following the rib concavity. Thus it "May" be it could still be described as "Engine Turned" but in a different manner & with a different pattern than the flat rib.
