It may be the angle of the photograph, but the stopper seems to lack quality (Wobbley) and it reminds me of Empoli bottle decanter stoppers.
Yes, I think this is definitely an Empoli stopper - I had a huge dark green decanter with squares, marked clearly Italy on the underside, and the stopper was exactly the same as this (and as wobbly).
The question is whether the decanter and the stopper belong together, I think possibly not.