The "swirls", are tool path marks replicating the cnc program
a nice big, deep, spiral cut. Engine turned marks on the mating surface look good and serve a function but the concentric spiral is still a mystery to me. They look like cast into the underside of the cover and maybe it is to add rigidity. They also would let the oil that splashed up onto the covers to drop back down more randomly rather than run toward the edges.
