Being half tanned, the leather is very tough and quite stiff so it doesn't really need a liner.
On the latest picture you can see a copper ring attached near the top, this is primarily for hanging the knife but it also ensures that the edge cannot possibly cut the stitching at it's most vulnerable point. Once past that, the shaping of the leather guides the blade safely into it's correct position.
I have never bothered with welts and to date have never had a problem with the blade slicing the stitching.