To a great extent I agree with you Christine, there is inevitably splitting of hairs here. However, we have to ask ourselves if there is any point in archiving pictures of labels, marks and signatures, unless we are going to use them to help us identify a piece of glass we either have or want to buy.
In the end, it all comes down to the integrity of the seller, and the knowledge of the buyer.
Most pressed glass isn't signed with a signature anyway, so it is primarily hot worked, or hand blown glass that is the issue here, therefor a recognisable Seguso piece which only bears a worn but recognisable label would surely fall into this category?:
3.a.a distinctive mark, characteristic, etc, that identifies a person or thing.