The S4VB is an early indirectly heated RF tetrode. For internal details refer to type S4VB.
The S4VA and S4VB specimens are typical of earlier Mullard oxide-process AC mains types (early to mid 1930s). The unsprayed S4VA has a less sophisticated form of assembly than the S23 and still relies on hand assembly using glass bead insulators. The internal skirt connected to the screen grid is a prominent feature of the design.
The reverse of the envelope is plain metallic paint.
The top of the envelope showing the Bakelite top cap skirt. The screw-on cap itself has been lost over time. The move to the top hat style of top cap happened around 1933-4.
The balloon envelope is 48 mm in diameter and, excluding the B5 base pins, is 121 mm tall.
References: 1931 data-sheet & 1043. Type S4VB was first introduced in 1930.