Hi Markus,
unfortunately I´ve got no idea which glassworks, but the mark is definitely upside down.
The sign of infinity combined with the cross is the alchemist´s sign for glass. It´s therefore
been used by several makers like Skruf or Fachschule Zwiesel...
Like always a picture of the whole piece won´t do harm.

edit: crossing... Well, in fact I´ve learned that from one of Ivo´s posts... LOL :hi: