This beautiful tube still has the original paper label. Visually the electron gun is obscured by the label but the base of the Y plates can be seen under the colloidal graphite coating of the bell. The tube has no connection for post deflection acceleration and so was designed for operation with about 800 Volts across the tube. The phosphor produced a green display.
Thanks to Jeremy Skertchly for identifying this CRT and supplying the fact that the 23D was used in the Cosor 1039 mini service oscilloscope. The latter was introduced in 1952 for TV service engineers to carry with them. This scope used three 6AM6 valves and one SD61 diode.
The attached data-sheet is of the oscilloscope by courtesy of John Evans of The Valve Page. This excellent site now has most of my collection of oscilloscopes.
The screen is not flat as glass technology at the time could not produce a flat end that would resist the air pressure when in tension. The dome ensures that the glass remains in compression all of the time.
The Cossor identification and the pinch.
The serial number is H20 and the tube was made in England.
The label wraps the neck of the tube. Note the BVA logo.
The end window envelope is 41 mm in diameter and, excluding the B12B base pins, is 180 mm tall.
Reference: Data-sheet. Type 23D was first introduced in 1952.