This was recently discussed on the Great War Forum and the consensus was that nobody had ever seen a definitely authentic WWI LE sling that was ink marked on the webbing - they were only stamped on the brass ends. I too wonder why WWI LE slings are so scarce other than perhaps almost all of them were used to destruction.
The cheaper display option is to find a sling with some natural age to it that has no legible post-WWI markings.