Seems to be any kind of carbon lamp to be series connected to form a Christmas or another decorative use garland. Obviously the presence of two bases in opposite ends of the bulb could confuse anyone because the absurd placing of them to be screwed to two lampholders at a time. But if we consider that this particular bulb could be used to be series chained, there is no problem to screwing wire attached lampholders to both sides one after another. Obviously the number of lamps used to make the garland could be in accordance with its own voltage drop and the total voltage at which the set could be connected.
Is the lead of each end connected to the thread and central tip at a time or instead is only the middle contact used for powering purposes while leaving insulated the threaded piece of the cap?
About vacuum tip, have you observed if it is inside one of the caps?, perhaps hidden by the adhesive mastic that sticks the glass bulb?. Obviously there is no indication about nominal voltage nor current.
Anyway nice lamp, good looking and its filament seems to be intact.