The DM70 is a miniature tuning indicator designed for dry battery operation. The valve is badged as a Mullard product but also as made in Holland. This implies that it was in fact a Philips designed and built device.
The display changes from minimum to maximum for a 10 Volt change on the control grid.
The display travels within the keyhole shaped column.
The left hand diagram shows the glowing column when the grid is at filament potential. The right hand diagram shows the effect of making the grid several volts negative of the filament.
The thin glass tube envelope is 9 mm in diameter and, excluding the B8D base wires is 41 mm tall.
References: Datasheet, 1040 & 1043. Type DM70 was first introduced in 1952.