The 12K5 is a space-charge tetrode or bi-grid valve, designed in the mid 1950's for car radio applications that could use the car battery as the anode supply. The design function is as a driver stage. Probably the audio output stage would have been solid state but transistors at that time did not have the HF performance to oust the valve.
Brimar describe the 12K5 as a miniature output tetrode, but again as a driver stage. Both data-sheets give the space-charge grid (g1) a current of 85 mA. The output of 35 mW would be far too low to hear even in a car with the engine switched off.
The anode side view. Note the carbonisation for heat transfer.
The grids can be seen clearly in this photograph. They pass flat across the wide cathode.
The anode face view. below the lower mica can be seen the copper grid supports, cathode and heater connections. Note the substantial nature of the inner grid support.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 59 mm tall.
References: Datasheet, 1040 & 3002. Type 12K5 was first introduced in 1956.