You are absolutely right Paul, as far as I've managed to work things out. Many of these are not marked and I originally bought them because I liked them, and thought they might be Mdina - when i started collecting it, nobody really knew what it was and i simply went by my instincts - which included this. Eventually, i realised they came in shapes which didn't ring true for Mdina, then i found a couple which were marked.
Some of the pieces do not have that lovely inky blue colour in them, but the distinguishing feature seems to be the greeny/browny/yellowy almost iridescent mottled blobs inside the glass, this is not a Mdina feature at all.
I cannot say anything at all about who might have made any marks on tham, I know no-oooothing about this at all. I don't think I've seen a mark like yours before.
The only info I have at all about Paul and marks is from Ron Wheeler, who contacted him to ask about artist signatures on Mdina and if any of them were made by him, the answer was a categorical and resounding no.
It is a lovely piece, I've got one a bit like it, and another couple, not quite so nice - no inky blue.