The POVT107 also known as the VT No. 107 was made for the British General Post office or GPO who had the state monopoly on telephone traffic in the UK at the time.
The POVT107 is an output pentode based on the M-OV N43. The load impedance required is 5,400Ω. The cathode resistance is 90Ω and in single ended configuration the output is 4.5 Watts.
The No. 107 can be seen on the right of the image.
The box anode is blackened to improve thermal radiation. The control grid is wound on copper supports for efficient heat transfer to the radiating fins above the top mica. On the right of this image can be seen the lettering for M O Valve Co.
This enlarged image shows the etched lettering more clearly.
The top mica: the top cap wire can be clearly seen to connect to the control grid.
The classic envelope is 56 mm in diameter, and excluding the B7 base pins is 133 mm tall.
Reference: Data-sheet. Type POVT107 was first introduced in 1936.