This exhibit is the Mullard ECC82. Designed as audio and general purpose triodes, two independent valves exist in the envelope.
An amplification factor of 17 is quoted, which is much lower than that for the pin compatible ECC83 at 100.
In television receivers the ECC82 was used as frame and line timebase multi-vibrators, AGC circuits and driver stages. It was as multi-vibrators that faults would most often develop.
Another ECC82 in the collection shows a different style of lettering in this long lived device.
The thin glass tube envelope is 20 mm in diameter and the length excluding the B9A base pins, is 48 mm.