http://www.authentic-campaigner.com/forum/showthread.php?t=2802&highlight=enfield+sling

Try and check out the above link for an indepth discussion of Enfield Slings. You may have to register to view it. If you go through all the pages and filter out everything you don't want to know, I think you will arrive at the conclusion that the sling with just the leather thong is post war and the CW slings had the brass D-buckle. There is also photographic evidence to back that up. I did a little survey once asking who had found the D buckles and where. It seems like they have been found in both theaters (east and western) and all periods of the war. The one thing you have to remember about Enfields is that they were made by a lot of different contractors and came with many little gotchas. Just when you think that you have it all figured out, something different shows up that proves you wrong. Bottom line is I would go with the sling with the D buckle.