The GEC E4205/B/7 and the Mullard ECR30 are small electrostatically deflected cathode ray tubes with a medium persistence green phosphor. Both were designed for oscilloscope use.
As with most of the CRT's from this period the heater-cathode insulation is not good and the maximum voltage differential between them is limited to 50 Volts. In practice with the cathode at about -800 Volts, the heater would be fed from its own transformer winding.
The a1,a3 voltage is given as 800 volts and the focus anode a2 would be held at about 135 Volts.
The GEC logo on the paper label wrapped round the 40 mm diameter tube neck.
This seems to be stating the obvious!
The black Aquadag colloidal graphite coating provides the final anode. The phosphor coats the tube face and the rounded end. Glass technology at this time was not strong enough to form a true flat face to the tube.
The B12B base has a key slot for the substantial locating spigot of the tube.
The base pins with helical slits to allow flexing to grip the tube pins.
A side view of the base.
The end window envelope is 70 mm in diameter, and excluding the B12B base pins is 170 mm long.