The CV1863 is a full wave rectifier based on the commercial 5Z4G. Professional equipment, including radar, etc., sometimes required circuits in which valve cathodes carried large signals and had to 'float' at voltages well below that of the chassis.
Designed to supply equipment of up to 40 Watts total power, this rectifier was made by several manufacturers. This suggests a very popular type, and one that found use in military equipment during WWII.
The CV1863 was also the designation for the 5Z4GY rectifier with the moisture resistant base for airborne and tropical use.
The pair of anode structures are seen end on.
This label appeared on another box and is in fact four separate labels that had been added over time. The box carried the crown and A M for an RAF valve.
The classic envelope is 42 mm in diameter and, excluding the IO base pins, is 100 mm tall.
References: Data-sheet & 1040. Type CV1863 was first introduced in 1935.