The SD20 in the picture is incorrectly identified as a battery valve. The SD20 is a 180 mA series heater chain valve for universal (AC/DC) sets. Released the previous year and in the Mullard data book is the SD4 that was a mains valve with the British standard 4.0 V heater for AC mains sets with mains transformers.
The screw connection at the top cap was an early design and this valve dates from the early 1930s but not before 1933 as it has a B7 base cap.
Apart from the E the reverse is plain screening paint.
From the top the close wound screening cage can be seen.
The balloon envelope is 49 mm in diameter and, excluding the B7 base pins, is 125 mm tall.
References: 4024 & 1043. Type SD20 was first introduced in 1934. See also1934 adverts.
