The VU39 is a pre-CV register name for a full wave rectifier used by the RAF. This exhibit has been rebranded by the reseller and is probably the product of the government surplus market.
This valve when used by the UK Government was labeled VU39 for the RAF or NU17 for the Navy or AU3A for the Army or even the POVT142 when purchased by the GPO. The rationalisation of the 1941 CV (Common Valve) register called the specification the CV1039.
The specification was based on rectifiers from the major manufacturers:- MU12/14 from M-OV, UU5 from Mazda, IW4 from Mullard etc.
The Bentley label.
The two mica discs with the twin rectifier sections between.
The face of one rectifier anode. Below the lower mica can be seen the ribbon connection to the cathode and the insulated heater. The latter passed four times through the cathode tube.
The classic envelope is 44 mm in diameter and excluding the B4 base pins, is 103 mm tall.