The CV1331 is a battery operated variable μ screened pentode. It was used as an RF or IF amplifier in portable radio receivers/transmitters. The direct commercial equivalent is the Mazda VP23.
The reverse is plain screening paint.
The filament can just be seen. It is a single inverted V and held in the hook of a helical tensioning spring.
The wide glass tube envelope is 30 mm in diameter and, excluding the MO base pins, is 82 mm tall.
References: Data-sheet & 1043. Type CV1331 was first introduced in 1938. See also1938 adverts.