The S21 is a screened grid valve (tetrode) from M-OV. It has a lower gm than the S22 and was aimed at sets with multi-stage HF amplifiers. This example has lost the anode screw type top cap used in the early 1930s.
The etched identification with the all enclosing screen grid with mesh sides. The anode consists of two bright plates placed either side of the central structure.
The Osram consumer brand paper label.
The other side of the screen grid. The control grid and filament cannot be made out.
The anode plates are supported at the lower end by wires held in glass beads. Other wires from the beads are secures to the screening skirt.
The balloon envelope is 43 mm in diameter and, excluding the B4 base pins, is 104 mm tall.
References: Datasheet, 1043. Type S21 was first introduced in 1931.