The DG4-1 is badged as Mullard but was made by the parent company Philips in Holland. The tube was listed as current production in 1956. The DG4-1 was designed for oscilloscope service and has a medium persistence green phosphor.
The DG4-1 has the same characteristics as the DB4-1 with a short persistence blue phosphor and the DP4-1 that had a short persistence blue phosphor combined with a long persistence greenish-yellow phosphor.
The first anode voltage is given as 250 Volts the second anode would have an average voltage of 800 Volts. At cut-off the grid would need to be at - 50 Volts. The sensitivity of the deflection plates is given as: x plates 130 mm per Volt and the y plates 210 mm per volt. This accords with standard practice of placing the y plates closer to the gun for a higher sensitivity.
The screen is 42 mm in diameter.
Excluding the B9G base pins the tube is 141 mm long.
Reference: Datasheet. Type DG4-1 was first introduced in 1953.