The DG2 is a bi-grid mixer and the design dates from 1932-4. It was described as a combined oscillator and mixer and thus made a saving of one valve. The mixer is an essential component of the superhet receiver.
A DG2 sold on ebay had a clear patch on the glass that revealed a horizontal box anode.
The identification is etched into the dome of the bulb. This image has been enhanced to reveal the lettering.
The balloon envelope is 46 mm in diameter, and excluding the B5 base pins is 92 mm tall.
References: Data-sheet & 1043. Type DG2 was first introduced in 1931.