The EL131 is the 35 Watt anode dissipation version of the 25 Watt EL34 from Mullard, they have the same pin connections on the IO base. It is believed that only 16 of these valves were ever made. This example came from the R&D lab at Blackburn.
The control grid has generous heat sinking attached to the copper supports. Also the screen grid is supplied with cooling fins.
The grey box anode is stitched on both sides.
In this image the twin getter holders can be seen at the top of the glass.
Here the EL131 on the left is paired with an EL34 on the right.
The anode seam and grid heatsinks.
Thanks to Rodney Trotter of Tellytone Amplification for supplying the pictures and data.
The wide glass tube envelope is 39 mm in diameter, and excluding the IO base pins is 115 mm tall.
Reference: Datasheet. Type EL131 was first introduced in 1955.