This exhibit is a CV valve that has lost the Type designation identification. The General Post Office was a UK Government body that ran the telephone system and controlled access to the electromagnetic spectrum.
The lettering reads: GPO Batch Sampled 5032 and so looks like a quality control item.
The pin connections follow the standard for low frequency B9A based double triodes.
The KB/DA identifies this valve as being made to specification K1001 or K1006, the B denotes qualification by a UK authority and the DA identifies the maker as Mullard Radio Valve Co., Blackburn.
The twin triodes.
The folded anode box is small with the metal stitched together. The KB/DA is still present but the rest of the information has rubbed off.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 48 mm tall.
