The SP2 is a directly heated battery pentode from 1934, designed for broadcast radio reception. The SP refers to it being a straight pentode with a sharp cut-off rather than the VP valves that were variable μ pentodes. The VP2 is an example.
We also have a later SP2 with classic envelope and top hat top cap.
The construction is hidden by the external metal paint but the SP2 is built like the early screen grid valves in that the suppressor is a substantial cage in the same way that the screen grid had been in prior years. PM12 illustrates this type of build for SG valves.
The reverse is just plain Mullard gold coloured zinc paint.
Between the top cap and the metalising paint one can glimpse the internal structure.
With extra light the three grids can be seen. At the top and in shadow is the small plate that forms the anode. The other side has a similar anode plate.
The balloon envelope is 48 mm in diameter and, excluding the B7 base pins, is 114 mm tall.
References: Datasheet & 4024. Type SP2 was first introduced in 1934.