It's possible that it's Caithness, although the hundreds of Caithness paperweights I've seen all have some sort of signature, or scratched numbers, or style desciption in boxy lettering on the bottom.
Those elongated, exaggerated bubbles are similar to Caithness Sea Dance Sable and some Moon Dance designs. And Caithness did make some magnums, which would explain the size and heft.
You might contact Caithness via its website.