The (RAF) VT25 was based on the landmark MOV type DET1 introduced in the later 1920s. Later developments replacing the thoriated tungsten single inverted V filament with a robust oxide coated filament of M shape gave increased power and reliability and became the DET25 (full details with this exhibit).
Changing to the M shaped oxide coated filament gave increased emission and changed the characteristics of the valve. In consequence the other electrodes were changed by M-OV to get back to the desired characteristics and that explains the move to the large box anode shown in the right hand valve above.
Both types of VT25 were widely used by the RAF for aircraft transmitters in the HF band. Power output was modest (around 30 W carrier) but the valves were economical, reliable and robust in their later form. The giant L4 base cap allowed generous insulation for RF anode voltages in excess of 1,000 V, even when damp (flying through cloud) or at high altitude.
During the 1930s and throughout WWII the RAF bought quantities of these valves from several manufacturers (including American). Exhibits VT25 and VT25 are both DET25's of MOV origin (but probably made at their Shaw shadow factory during WWII rather than at Hammersmith). Exhibit VT25 is of characteristic Ediswan design but was presumably made in an Ediswan Mazda factory which in peace time had made valves for the domestic market under the Mazda label.