The DE3 is a general-purpose dull emitter triode that has a very delicate thoriated tungsten filament rated at 2.8V 0.06A.
The DE3 was one of the first British valves intended for operation from dry batteries but the battery required was a 4.5V 'bell' battery. This was used with a series rheostat adjusted by the user to provide approximately the correct running voltage across the filament terminals.
1.5 V cells made in the 1920s soon dropped to 1.0 to 1.1 volts, due to polarization, when used continuously as in a radio.
In addition to the paper band around the envelope the type designation is etched into the glass.
Here we have a closer view of the DE3 and the other etched markings.
The wide glass tube envelope is 25 mm in diameter and excluding the B4 base pins, is 94 mm tall.
References: 4051 & 1003 Type DE3 was first introduced in 1924.