That is curious - the type of stopper is exactly what Biot uses for their decanters. Except, of course, Biot glass all has bubbles and a circular blowpipe finish for pontil mark. And it is stamped.
Realistically, it is not the quality of glass which s at issue, but the finish - if a company takes the trouble to properly finish and polish rims and pontil scars, then they are in a higher league. Which is an identifyer in itself.