The DE3 is a dull emitter triode from 1924 and this exhibit is from later production and has a moulded Bakelite base and a small balloon envelope. M-OV sold the DE3 under both the Marconi and Osram brand names.
The filament is rated at 2.8 Volts at 0.06 Amps, this suggests two dry cells in series as the filament supply, but in fact a 4.5 Volt 'bell' battery would be used with a series rheostat. The filament could thus be maintained at the 'correct' voltage. The valve can be used at LF or HF or as a detector according to the label.
The obscured construction is of a vertical anode 18 mm long and 5 mm diameter, into which the grid and filament fit co-axially. The filament is a single strand, and the grid is a fine wire helix supported on a side rod through the whole length of the anode.
The envelope has etched into it the Marconi Valve on one side and this side has the Type designation DE3 at the top with 2.8 V below and finally the type two BBC stamp. The stamp places the manufacture of this exhibit to between September 1924 and 1927. The Bakelite base suggests the latter end of the date range.
Another view of the BBC stamp and the electrode assembly.
The balloon envelope is 34 mm in diameter and, excluding the B4 base pins, is 72 mm tall.
References: Datasheet, 4051 & 1003 Type DE3 was first introduced in 1924.