That's interesting, I have seen a few of these signed and also plenty that were not. Often the signature is very faint and difficult to make out. My first reaction was that the signature was added later as it 'usually' (as in Wayne's last photo) looks like P . Strom rather than P O Strom (the O is so small it ends up as a . ). However, as Ivo has another signed in the same fashion then I too would expect them to be genuine signatures.
I would not necessarily agree with a glassworks being too obscure for someone to try forging a signature, those who are dishonest often will try almost anything to make more money. Usually their knowledge is limited and it is not as if they care, all they want is profit.
Today I saw a faked Mdina signature on a nice enough and genuine bit of Mdina at a bootsale, I would have bought it but it was just too damaged (a couple of nasty impact marks).