O-D;
I have wondered about that on my rib myself, but of course have no way of knowing. It would have had to have been very light I believe for no traces to be left, but if that's the way most of this era were finished this one probably was too.
That on yours looks much like the same patern as the later flat engine turning. The cuter would have to be set on an angle & cut one direction only, then tilted to an opposite angle & the other direction cut. This actually appears as if a very small dia cutter was used & more than one pass per side. The flat ones could be cut in one pass I think.
