So, for those interested in reconstructing an official Austrian Military spec. airgun/airtank case - and who isn't? - a key question about spec. 5023 is exactly what is Terzenleder? As it turns out, Terzenleder is oxhide produced from oxen 2 - 3 years old. So, Terzenleder is a heavy duty, stiff leather.
S. 5023 is a heavy-leather case.