The leather tie thong at the bottom sling swivel is very hard to see in period photographic evidence. I have not yet seen a period WBTS photo that clearly shows this tie thong, but that does not mean it was not a feature of the British slings that found their way to both North and South. This tie thong IS clearly seen in period photos of British Empire troops armed with Sniders, which used the same slings as the P53-series of arms.
The brass buckle is more commonly seen in period photos than the standing leather loop, as an adjustment feature. But again, most period photos of soldiers armed with Enfields don't show either one - you just can't see that part of the sling in the image.
I would also recommend the Enfield sling with the brass buckle (correct style - find a repro with a buckle based on an original), and the thong tie at the lower swivel.