JBR -PA24
I believe the finish you are refering to is made with a single point tool that is sort of like a cheap hole cutter for wood. The rib is mounted on a mill with a long bed and the feed rate is set for a constant advancement the cutter rotates as the rib passes making a convex cut ont the frount of the pass and a convex cut on the back. When everything is set corectly you get a very nice effect. See a DM Lefever or an Ithaca flues.
bill