The 6K7 pentode was designed for intermediate frequency amplifiers in 'modern' superhet type receivers during the mid 1930s. A couple of years later the slightly changed single ended version 6SK7 appeared.
It is high impedance allowing for a high Q in the tuned anode circuit. It has a long grid base allowing for the stage gain to be altered according to the grid bias applied. Thus this a variable-μ valve where the gain is to be controlled by and AGC voltage developed at the detector stage. This was usually an additional diode. Typically within a double diode triode such as the 6Q7.