This is a page from a 1921 Elliot Lewis Electric Co. showing G.P.Co. tubes. I have never seen one and marked ones are probably scarce. Since G.P.Co. owned the G.F. Brunt plant and used the Brunt trade name on their knobs, cleats and tubes until how long, I assume that's why G.P.Co, marked electrical items are hard to find. I have a few G.P.Co. marked knobs, spools, knife switch and a couple of sub bases but no tubes. Have to keep on searchin. Take care stay safe and be well to all.