The DFA range of triodes was designed for the output stages of broadcast receivers. The box describes the DFA1 as for power amplification to drive the largest loudspeakers. In 1924 when the DFA1 was issued the speaker would have been of the moving iron or reed type and probably fitted with a horn.
The paper label on the nickel base shell describes the warranty conditions and then gives the filament and anode ratings.
The envelope carries the third style of BBC stamp that dates this exhibit to between September 1924 and 1927. With the metal base shell this valve is most likely from 1924 or 1925. The base has the Mullard name embossed on it.
The Mullard logo with DFA1 and the words Registered Trade Mark.
The balloon envelope is 53 mm in diameter, and excluding the B4 base pins is 99 mm tall.
References: 1003 & 1043. Type DFA1 was first introduced in 1924.