We haven't a clue.

I do not for one moment think the 73 is a year, this design is later than that, although it does appear with a paper label with biro writing of odd numbers on it as well as the paper Mdina label, but I have also seen this design with plastic labels (The bark textured cylinder vase is one shape which has been resurrected in this design and is what I have seen with a plastic label on.)
I do like it!
The deep purple glass used to create the black is a completely different colour to the early amethyst glass used.