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).
As one would expect this is a German made valve.
The anode is shaped so that the central section is close to the grid and the outer part is able to radiate away the heat generated in operation.
Below the lower mica is a shield.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 48 mm tall.
References: Datasheet & 1040. Type ECC88 was first introduced in 1958.