there is a whole thread here somewhere on this topic. Don't have time at the moment to search for it. There are some excellent period (yes, B&W) pics on the WNW site of the cowls. I think the "purpose" was simply to give the parts a quick finish, as they were hand beaten and if simply affixed to the plane after coming off the shot bag they would look crude and sloppy. If you've ever done panel beating work it makes sense. I think it was done with a wire wheel and probably only took a few minutes at most.
