The E2599 is an experimental triode. The the flat rectangular cathode is connected by tape to four base pins - indicating use in the grounded cathode configuration. The anode is a U channel with a flat working face. Between the flat faces of the anode and cathode is a frame grid, indicating a high mutual conductance.
The pin connections match those of the GEC A2599 type of valve and this is probably the prototype of the production A2599.
The anode is on the right and the grey metal visible is one half of the cooling arms
Looking directly at the frame grid with its gold colour. The cathode is the rectangle behind the centre support. The grey of the anode fin can be seen through the left side of the grid. The full size of the original image is required before the grid wires start to be visible.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 48 mm tall.