The SP220 is a battery audio output triode from the early/mid 1930's.
As a low power device the anode has been left bright and even with the grid acting on both sides of the filament the anode box has a small front to back dimension.
The identification is carried on the dome of the envelope in etched form.
The filament is oxide coated and consists of two inverted V's. The tension springs can be seen in the centre of the image. The side micas are designed to be held in the shoulder of the envelope to reduce movement and this microphany.
The grid can be seen when looking up from the base. The tungsten filament wire is held in folded supports.
The classic envelope is 44 mm in diameter, and excluding the B4 base pins is 100 mm tall.
References: 1040 & 1043. Type SP220 was first introduced in 1933.