Data from the Glass Study
1907 directory Glashütte vormals Gebrüder Siegwart, Stolberg, b. Aachen (Rheinpr.) I think there is an advert for these too but not digitised that yet.
1925 directory Glashütte vorm. Gebrüder Siegwart & Co., Stolberg, Rheinland.
also Glasfabrik vorm. Gebr. Siegwart & Co., A.-G. 109. as a subsidiary of Aktiengesellschaft der Spiegelmanufaktur und chem. Fabriken von St. Gobain, Chauny & Cirey.
Products:
Fabrikate: Spiegelglas in allen Stärken, zum Belegen und Verglasen; silberbelegtes und facettiertes Spiegelglas; Rohglas aller Art und Stärken; Drahtglas; Ornamentglas; Klarglas; Prismenglas; Waschbrettglas; gebogenes Spiegel-, Roh-, Draht-, Ornament- und Fensterglas. Spezialität: große Spiegelglasscheiben. — 10 Oefen, Kraftmaschine ca. 6500 PS. 1100 Arbeiter. 143 Arbeiter- und Beamtenhäuser mit 245 Wohnungen. (Gegr. 1853. )
It also shows the trademark.

1945 data with war time production:
Vereinigte Glaswerke, Werk Stolberg
Stolberg, Aachen
Date of Visit 8th August, 1945, 23rd September, 1945
Products
Plate Glass (Rough Cast and Polished)
Rolled and figured glasses
Wired glass
Bent glass
Special white glass
1878 report states that St Gobain set the works up to produce Plate Glass for the local market.
So where does Schmid come in Pamela? All we have in the Study is from 1878:
Jos. Ed. Schmid, Annathal.
The usual variety of colored glass fancy articles. This house also exhibited a style of crystal heavily cut hanging baskets, with handsome cut drops, which were in very good taste and beautiful. Many small bouquet-holders in iridescent glass, blown very thin, had heavy solid feet; the display of the iridescent colors gave these goods quite a pleasing appearance.
The only other glassworks I have listed in Stolberg, made mirrors.