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).
The triode sections are threaded with a Z shaped screen plate.
With excess front lighting the shape of the depression in the anode can be seen clearly. It is only the bright surface indside this depression that acts as the receiver of electrons the rest of the metal is present to radiate heat away.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 47 mm tall.
References: Datasheet & 1040. Type ECC88 was first introduced in 1958.