The GDK6 is a miniature sharp-cutoff pentode designed for use as a wide-band radio-frequency or intermediate-frequency amplifier in television receivers. Features of the tube include high trans-conductance and low inter-electrode capacitance.
The construction is obscured, but the shield can be seen under the lower mica.
The General Electric logo.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 47 mm tall.
Reference: Datasheet. Type 6DK6 was first introduced in 1958.