Mullard describe the SP4 as an indirectly heated HF pentode for use as a speech detector, or RF amplifier, in AC mains receivers. The B5 based version was introduced in 1933 and the B7 based version a year later.
This exhibit with the classic envelope looks to be a later production and the missing top-cap was probably the top hat style rather than the earlier screw top as seen inSP4.
The heating time is given as 35 seconds. Equally important is its relatively thick heater (1 Amp rating) and improved heater-cathode insulation.
Apart from its ungainly size, the main weakness of the SP4 was that it was unsuitable for AGC applications and therefore had no place in the 'modern' type of superhet which became the norm from the mid-1930s onwards, using the variable-mu pentode for RF and IF amplification.
The classic envelope is 43 mm in diameter and, excluding the B5 base pins, is 103 mm tall.
References: Datasheet & 1043. Type SP4 was first introduced in 1933.