The SP13 is a straight RF pentode and in the Mullard version of the SP13 it has a Ct8 base and the SP13C has the B7 base. The Tungsram version has the B7 base on the SP13 and the optional Ct8 version is known as the SP13a. Basically they are all the same valve electrically.
Functionally this valve was for use in universal (AC or DC mains) sets and also in car radios.
The outer screen connects to top and lower screens for an almost complete enclosure.
The lower screen with gaps for the electrodes. The insulated heater passes through the cathode eight times.
The classic envelope is 38 mm in diameter, and excluding the B7 base pins is 98 mm tall.
References: Datasheet, & 1043. Type SP13 was first introduced in 1934.