My first thought was a 'Bakelite' knob, but then I read that you said that it's glass.
The presence of a makers name, means that it was likely a patented design or mould on the piece.
Hopefully one of the Tnet members will recognize the name on the knob.
Wow interesting stuff. Thanks. I could see it being a stopper. And also it saying Hoffmann, though the upside, inverted, backwards....is odd. Hoping someone out there knows more