The 154V is a Mullard indirectly heated (4V AC) mains triode that was designed before Mullard's had a satisfactory oxide coating technology and were still largely dependent on the Azide process. Because the azide process broadcast a barium coating everywhere within the anode structure and not just on the cathode surface, it was necessary to design the structure so that all insulating (ie. glass) surfaces and supports were well shielded from flying barium. The classic horizontal arrangement achieved this but made it very difficult to assemble valves with close grid-cathode spacing. Early Mullard indirectly-heated valves therefore had relatively coarse spacings and, as a result, low amplification factors.
The side terminal has lost the original bakelite cap. The paper label gives the essential operating details.
Looking up under the silver gettering the box anode, grid, coated cathode and insulated heater can be seen.
Another view showing the mounting of the electrodes above the pinch.
The far end opposite to the connections.
The balloon envelope is 49 mm in diameter and, excluding the B4 base pins, is 85 mm tall.
Reference: 1043. Type 154V was first introduced in 1929.