The Pen46 is an early television line output pentode valve. In line scan duty high surge voltages appear, this valve can withstand an anode surge of 3,000 Volts.
The box anode is open at the sides, and two of the helical wire grids are visible.
The classic envelope is 50 mm in diameter and, excluding the MO base pins, is 118 mm tall.
References: Datasheet, 1040 & 1043. Type Pen46 was first introduced in 1940.