The L21/DD is a battery double diode triode for use in superhet receivers. The data-sheet makes it clear that the diode anodes have specific uses. The reason is the difference in potential from one side of the filament to the other.
The data-sheet lists details for the similar HL21/DD.
A maker's code within the circle.
The crown and AM demonstrate that this exhibit was made for the UK's Royal Air Force.
Little beyond the top mica can be seen.
The classic envelope is 38 mm in diameter, and excluding the B5 base pins is 103 mm tall.
References: Datasheet, & 1043. Type L21/DD was first introduced in 1934.