The DEQ was introduced in 1923, and was intended to replace the Q and QV. The construction is similar to the Q. The filament is a single wire running the length of the envelope and tensioned by a spring at the end opposite the anode and grid supports.
The DEQ is a very high impedance valve and its primary function was as a detector.
The details on the envelope are all etched into the surface. The Marconi Valve name appears with Made at the Osram Lamp Works. The Type DEQ is present as is the number 171xx the final numbers are covered by the green paint band. Finally there is the BBC Stamp.
The filament tension spring end. The brass end caps are corroded and a meter reading says the filament is open circuit. However, after cleaning some corrosion away the filament springs to life.
The at thos end the glass fork provides support for the filament, anode (top) and control grid (bottom).
The Words on the BBC Stamp are Type Approved by Postmaster General. This type one stamp was used from November 1922 until September 1924. As the DEQ was introduced in 1923 we can date this valve to within a year.
The thin glass tube envelope is 19 mm in diameter and, tip to tip measures 74 mm.
References: 1003 & 1004. Type DEQ was first introduced in 1923.