Fascinating - never seen anything like those before. Given the fabric covering it would surprise me if there were incandescent filament lamps underneath, due to the heat generated posing something of a hazard. Shine a bright torch (flashlight) through their sides and see if the colour shows up. If they appear colourless like that, there could be versions of the "fluorescent" lamps underneath, same technology as the more well known globe shaped Sylvania types. Don`t go taking a scalpel to the covering of one of them though, you`ve got something pretty rare there by the looks of things.
ETA:
Just noticed you said C6 size - means they are series burning types, and the fluorescent type lamps would not operate in series, they require a much higher voltage limiting them to parallel operation. That means there probably are filament lamps underneath. As you were....
