silly question.. but are you certain its a 338-416 and not a 416 Taylor?
i went and read a little more about the 338-416.. it looks like the only difference between the two is as you describe, the location of the shoulder... the 338-416 has a neck that is just a couple of thousandths longer, so the shoulder is just barely pushed back...
Im no gunsmith.. but Im thinking if you put a 338-416 in a 416 Taylor chamber, and "fire formed" the brass, you might get the strange looking second ring that youre seeing when the brass stretches to fill that slightly longer neck area and meet the taylor dimensions..
I'd be curious to see what an ejected 416 Taylor cartridge looks like after firing (assuming the above is correct and its the 338-416 shoulder that is further to the rear and the 416 T would chamber properly..)..