The D15 is a current stabilising barretter. The current remains sensibly constant at 150 mA for voltages within the range 90 to 140. The barretter functions only within a series heater chain. The cold resistance is 240Ω.
At this magnification the The filament looks to be is a single strand looped over the supports. Traditionally the wire is iron and the low pressure gas inside the envelope is hydrogen.
A closer look reveals the filament to be a coil.
Here the coiled filament is clearly seen.
The wide glass tube envelope is 39 mm in diameter, and excluding the IO base pins is 77 mm tall.