Hi Javi, I won't comment on that particular cover because I don't know enough about these covers, although there are a LOT of postwar Norwegian and Yugo items passed off as original German WW2, especially on ebay, but on dealer websites as well.
As far as the website you mentioned....it's a cool website, but there are errors on that particular site, I refer to Myrvang's book, and also rely upon a couple members on this forum for their expert opinions....I wouldn't use the GermanmilitariaUK website as my first reference.....