The Pen4V from Mullard is an indirectly heated audio output pentode. The above adverts give more details. The valve was originally made with a B5 base cap and side terminal. Thrower gives the advent of the B7 based version as the Pen4VA.
The mesh box shaped anode with all electrode top connection held in position within the glass rod.
The dome carries the identification, maker name and 4.0 Volt heater rating. Also the rectangular suppressor grid and shaped screen grid are better seen.
The pinch and lower grid windings.
Thanks to Mark Mallison for sending the images.
The balloon envelope is mm in diameter and excluding the B7 base pins, is mm tall.
References: Data-sheet, & 1043. Type Pen4V was first introduced in 1931. See also1931 adverts.