... the base looks solid with the ribbons of the crown meeting in the centre as they do on top.
The ribbons and spirals start at a point at the base then follow the sphere of the weight to the top where they end back in a point, thus creating a hollow centre.
There is definitely some confusion about what is meant by "hollow". It is known that St Louis, in the 19th century, made Crown weights which were truly hollow (i.e. they were blown as an open bubble). However, most other makers of Crown weights (19th, 20th and 21st centuries) used a solid dome to which the "twist canes" were applied and usually covered over with a further layer of clear glass.
Sometimes a truly hollow weight will retain its opening in the centre of the base. But in some cases, it can be closed off and finished neatly such that it is not obvious (apart from the weight feeling distinctly light in weight for its size) that it has air inside.
The same point has been made in the past about millefiori canes which have been described as being "hollow" - in fact, apart from one instance that I know of, all millefiori canes that look "hollow" simply have a clear glass centre.
My view of the weight in this thread is that is, as said by others, a Murano item (and not necessarily Fratelli Toso) with a solid clear glass centre - not truly "hollow".