The BTH B2 is a general purpose bright emitter triode that was introduced in 1923. Following the trend in design, the anode shape had moved away from the cylindrical shape to a box design. The orange colour is probably due to using phosphorus for gettering the valve.
The base is all composition rather than using the outer metal shell of earlier designs.
As with all bright emitters, the filament would be connected to the LT supply via a rheostat. In use the series resistance would be slowly reduced from the maximum until the output was acceptable. To maximise the filament life it was always sensible to run the filament at the lowest possible temperature.
In front of the anode box is a blob of glass. A strut from the anode supports the glass and the filament tension spring is embedded in the glass, the glass itself is a structural item and a good insulator.
Looking down into the anode the shaped grid and the bright tungsten filament can be seen. The top of the inverted V of the filament is held by the spring attached to the glass support.
The method of seeming the anode and the shape of the grid are clearer in this image.
The shape of the filament support is clearer in this picture.
The balloon envelope is 37 mm in diameter, and excluding the B4 base pins is 91 mm tall.
References: 1003 & 1004. Type B2 was first introduced in 1923.