The ECC88 is a VHF double triode with separate cathodes. The ECC88 was designed for service as a cascode amplifier in the front ends of VHF radio receivers. One triode acted as the local oscillator and the other as the mixer (frequency changer).
This exhibit is of later manufacture than ECC88 as can be seen by the logo. Also the construction is slightly different.
The top mica is quite thick which would increase the valve life in environments subject to vibration.
The getter holder is held from two sides. Note the screening between the base and the lower mica disc.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 47 mm tall.
References: Datasheet & 1040. Type ECC88 was first introduced in 1958.