My only real thought is that the stopper is not the original.
I can see an etched bit of the bottom part still sticking up over where it ought to be.
A very vague thought floating around is that it might have something to do with St. John's, a small company licensed to make maximals and minimals for IoWSG, but with whom things went sour when they tried doing more than they'd been licensed to do - or something like that.
I really do not know if they tried making things such as this, though.