The EL3 is an audio output pentode from 1938. The EL3 would typically be used in the audio output stages of broadcast radio receivers. The Ct8 base was much used in continental Europe but far less popular in the UK.
The reverse carries the paper label of Philips Italy.
The gettering at the top, the internal coating next and the screening paint lower down allow very little of the internal structure to be seen.
The dome with interference fit mica wings to hold the electrode assembly rigid within the shoulder of the envelope.
The classic envelope is 44 mm in diameter and, including the Ct8 base pins is 109 mm tall.
References: Datasheet, 1040 & 1043. Type EL3 was first introduced in 1938.