The 5591/403B is a physically small special quality amplifier pentode for use as a low noise amplifier at VHF and UHF and the design dates to the late 1940s. The 6AK5Wet al have slightly different heater ratings.
The supports for the three wire grids can be seen through the notch in the anode.
Here the two outer grids are very clearly seen. The control grid appears as small ripples close to the cathode in this image.
The thin glass tube envelope is 18 mm in diameter and excluding the B7G base pins, is 35 mm tall.
References: Data-sheet & 3002. Type 5591 was first introduced in 1947.