I finally found a reference that is very similar to the small glass wiring knobs that have been found in Belgium [id=794803980]. it shows that low voltage wiring threads between the "bumpers". This example was listed in a 1928 French electrical supply catalog. The "bte (short for brevete = patented). sgdg = Sans Garantie Du Gouvernement (without governmental guarantee).

Loose translation for this spool/knob publication says: "Spools fasten in hardened porcelain to stretch the wire rigid and supple quickly. Very practical for overhead industrial installations, of streets, of sheds, etc."