Here is a photo of a Skruf sticker.
The mark at the top of this thread, the mark I referred to earlier (in the books) and this sticker all have (what looks to me like) an infinity type symbol and a cross. All three marks vary but this is not uncommon as nearly all glassworks' identifying symbols evolve over time.
John.
I would photograph the symbols in the book but I think that would be breaking copyright, so best not to.