This 354V from Mullard has a balloon envelope and a horizontally mounted box anode assembly obscured by the silvering inside the glass.
The original 354V triode dates from 1929 and in 1935 the heater current requirement was reduced to 0.65 Amp. The 354V was the successor to the 484V.
The balloon envelope is 50 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.