This DER form M-OV has the same horizontal electrode assembly as the R Type. The early adverts do not show a spherical envelope for the DER but do show a horizontal electrode assembly.
Thanks to Nicola Lillis for sending these images.
The original box.
The filament has broken but the envelope carries the BBC Stamp with the words 'Type Approved by Postmaster General' and so dates the valve to between 1 November 1922 and September 1924.
The balloon envelope is 52 mm in diameter, and excluding the B4 base pins is 95 mm tall.
Reference: 1003. Type DER was first introduced in 1921. See also1921 adverts.
