The SP13C from Mullard is a screened amplifier HF pentode. The similar SP13 has a CT8 base. The control grid is taken to the top cap for maximum isolation between input and output circuits. This valve was used in RF and IF stages of domestic radio receivers with a common heater chain and used for Universal AC/DC sets.
The reverse contains the BAF logo.
The classic envelope is 42 mm in diameter and, excluding the B7 base pins, is 102 mm tall.
References: Datasheet, 1040 & 1043. Type SP13C was first introduced in 1935.