The filament is encased inside the tube, it was buried deep in the ground, my thoughts are its a very early bulb, i cant remember seeing one like it before, any older members who can recall them ?
Looks like an ordinary bulb with the glass missing. The two metal rods inside the glass should extend past the end of the long piece of glass. The filament is attached to those an held in place by two more pieces of wire attached to the end of the long piece.
The top edge of the bayonet connector looks as though there should be something in there, difficult to see from the angle the photo is taken from. Is there anything in there like glass or mastic? R-