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 the metalised valve we have here and was advertised as such up to 1947. Later Mazda realised that rectifier valves need good heat loss more than screening and the clear U403 was introduced.
Thanks to Mark from Vintage-parts for sending this image.
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.
Absolute Maximum Operating Conditions¶
Updated February 27, 2017.