sorry, can't really help you with any provenance or attribution, but wanted to offer suggestions on your comments re 'swirls' - although as a bit of a novice, I am quite prepared to be 'shot down' by the experts

I undertand that these vertical lines in the glass produce an effect that is called 'optical moulding' - something which can be seen, but not felt, from the outside of the piece. In your pictures, it is not apparent that the 'swirls' go from right to left - although when this does happen I believe it is termed a 'wrythen effect'. If I'm wrong on the next bit, I do expect to be put right very quickly...........in order to create these vertical 'optical' ribs, glass is blown into a mould that leaves an impression of 'ribs' on the
outside of the glass. In order to remove these indented ribs,
from the outside of the the glass the piece is then 're-blown' into a larger mould,
WHICH HAS A SMOOTH INTERIOR SUFACE, thus removing the ribs from the outside of the glass piece, and creating a smooth outside to the item. Of course, the inside of the piece retains some of the ribbing, thus creating the optical effect, but with a smooth exterior. The popularity of 'optically moulded glass' was very evident prior to the 1940's. Sorry, but other than that, I know 'naff all' (English slang colloquialism meaning I'm ignorant) about your glass. As far as the Belgian material goes, the only name that comes readily to mind is VSL.