The DW4/350 (DW4-350) full wave directly heated rectifier was introduced by Mullard in 1936. The specification is very close to the Mullard DW3 released in 1931. The difference in this later valve is a lower internal resistance allowing the output voltage for a 350-0-350 Volt input to be 380 Volts rather than the original 320 Volts.
The flat faces of the anodes are ribbed for dimensional stability.
The Type designation and maker's name are on the dome in a circle of gold paint. The rest of the glass is plain as metalisation on rectifiers was both unnecessary for shielding and caused the valve to overheat and fail.
Close-up on the oxide coated ribbon filament. Both sections are, of course, run in parallel as the filament is the cathode. The filament top support is provided by the two central rods that run the length of the valve.
The balloon envelope is 49 mm in diameter and, excluding the B4 base pins, is 114 mm tall.
References: Datasheet & 1043. Type DW4/350 was first introduced in 1936.