The A2599 is a low noise VHF triode for grounded cathode operation. The design is of the frame grid type with cathode one side and anode on the other. This planar construction and the multiple cathode connections place the assembly part way to disc seal construction but at considerably less cost.
See AlsoE2599 which looks like the prototype of this design.
The electrode assembly occupies only a small part of the envelope.
The GEC paper logo.
This view is looking sideways onto the planar triode. The cathode tube is on the left and looks bright as it is coated on the working face only. The gold plated structure is the frame grid. The black flat ended U channel is the anode.
Here the view is looking through the fine wires of the frame grid.
At a similar angle to the previous image but taken through the microscope. The side of the frame is at the top and the white area is the working face of the cathode.
The thin glass tube envelope is 21 mm in diameter, and excluding the B9A base pins is 46 mm tall.
Reference: Datasheet. Type A2599 was first introduced in 1960.