The N308 is a television line output valve. The deflection coils of a magnetically deflected CRT produce a linear increasing magnetic field when a voltage is applied. It is the job of the line output valve to act as a switch and thus apply a step pulse to the coils. At the end of the line scan the valve switches off and the magnetic field collapses generating a large voltage spike. This can be several thousand volts and so the valve must be well insulated, the N308 has a maximum positive surge capacity on the anode of 6,500 Volts. It is also why the anode is taken to the top cap.
The large anode is carbonised so as to be a more efficient radiator. The working surfaces are to the left and right of the slotted ends. The anode has a dissipation rating of 10 Watts and the screen grid can dissipate four Watts.
Looking through the holes in the anode one can see the bright beam forming plates and within that the inner grids and cathode. Both g1 and g2 pass flat across the working face of the cathode.
The wide glass tube envelope is 29 mm in diameter, and excluding the IO base pins is 89 mm tall.
References: Datasheet & 1040 Type N308 was first introduced in 1956.