The 28D7 is a double beam tetrode designed to work as a class A audio amplifier. When both sections are used in push-pull the output is 175 mW. The anode and heater circuits are rated at 28 Volts. This is the voltage one would expect to find in a lorry or truck when the engine was running.
The triode sections appear too large and well constructed to provide such a small audio output.
The Type designation is etched into the dome of the envelope.
The wide glass tube envelope is 28 mm in diameter, and excluding the B8B base pins is 64 mm tall.
References: Datasheet & 3002. Type 28D7 was first introduced in 1943.