The 6K7 pentode was designed in the mid to late 1930s for intermediate frequency amplifiers in 'modern' superhet type receivers. It is high impedance allowing for a high Q in the tuned anode circuit. It was finally made in the small Glass Tubular format we have here.
See also the 6K7G full sized glass envelope version, and 6K7 original metal valve.
The main image shows the Type designation etched into the glass and sitting within an octagonal border. This is the RCA format.
The valve is branded Mullard and Amerty. The Amerty brand was reserved for valves made in the US for Mullard.
Within the dirty glass envelope can be seen the three grid supports. The control grid is wound on copper supports.
The dirt that can be seen is what has remained after the valve was cleaned.
The wide glass tube envelope is 29 mm in diameter and excluding the IO base pins the valve is 73 mm long.
References: Data-sheet & 1040. Type 6K7GT was first introduced in 1938.