The U403 is an indirectly heated half wave rectifier designed for universal mains equipment. That is either AC or DC mains. The 200 mA heater is designed for series heater chains.
Initially the U403 was introduced as a metalised valve and was till advertised as such in 1947. This exhibit is of later manufacture when the benefits of a longer life through better heat loss were recognised.
The two anodes are joined above the top mica.
The pinch level components. Note the significent heater insulation.
The classic envelope is 38 mm in diameter, and excluding the MO base pins is 82 mm tall.
References: Datasheet, 1040 & 1043. Type U403 was first introduced in 1938.