This exhibit is a Cossor DD4 or DDL4 double diode. The DD4 has a higher maximum current than the DDL4. It is assumed that the main type identification was originally on a paper band fixed to the envelope above the base. The identification etched into the top of the envelope is V10.
Both of these Cossor valves date from the mid 1930's when detector diode for television reception were being developed. Low capacitance was a major design feature so as to be able to operate on vision signals up to 3 MHz.
The lettering is distorted by the mirror gettering on the inside of the glass but seen directly shows V10.
The flat anodes can be seen at this angle. The cathode is oxide coated.
The balloon envelope is 30 mm in diameter and, excluding the B5 base pins, is 78 mm tall.
References: Data-sheet & 1043. Type DD4 was first introduced in 1934. See also1934 adverts.