The EL33 audio output valve is the little brother of the 25 Watt EL37 with which it shares pin connections. Mullard describe this valve as a high sensitivity output pentode for use in AC mains equipment. The EL33 could be used in push pull to deliver 8 Watts, but the normal use was in domestic radio receivers as a single ended stage.
The internal coating obscures the inner detail.
There are two top mica sheets. The visible one has 'fingers' to grip the inside of the envelope. Blow the visible one is the mica that hold the electrodes. with a magnifying glass it can be seen that the control grid has heat radiating fins attached to the top of the supports. This exhibit has been cleaned but the photograph shows the remaining dirt.
The classic envelope is 44 mm in diameter and, excluding the IO base pins, is 109 mm tall.
References: Data-sheet, 1040 & 1043. Type EL33 was first introduced in 1938. See also1938 adverts.