The ECC88, 6DJ8, CV 2492 are high performance valves that have passed rigorous military specification tests regarding ruggedness, heat and shock resistence, and heater longevity. Like their civilian/industrial counterparts, they have the four moulded seams on the top, heavily plated gold pins, a halo top getter with a sputter shield below (with slight raised indentations at the 3 and 9 o'clock positions on the shield) which give rigidity and lower microphonics.
There is also the 7V 300mA heater version of the ECC88, designed for RF usage, the PCC88 or 7DJ8.
Mullard describe the E88CC as being for use in cascode circuits and as a multivibrator and cathode follower in computers. The special quality in this case referred to its long service life and not, as normal, to its being a ruggedised version. The ECC88 and the special quality gold pinned variants find use in many pre amplifiers and a surprising range of exotica use them:-