|A Cathodeon monoscope tube type C912|
This is the Monoscope tube, based on a CRT gun assembly and the target with the pattern on it in the bulb. The connection on the side of the bulb is for the anode. The pip on the far side is the evacuation seal.
This tube was made by Cathodeon is a
type C912 and the label says "Rediffusion London" which would
be the caption. Alas as you can see the vacuum has failed and the air
has got in RIP.