The Mullard 354V is an indirectly heated triode and has an external metallisation to act as screening.
The metallised balloon envelope.
The paper label warns about not earthing the metal paint as it is connected to the cathode.
The balloon envelope is 46 mm in diameter and, excluding the B5 base pins, is 92 mm tall.
References: Data-sheet, 4023, 4024, 4048 & 1043. Type 354V was first introduced in 1929. See also 1929 adverts.