This Mazda full wave indirectly heated HT rectifier uses the standard 1930's four volt heater rating. No primary information has been found but it is generally considered to be a 500-0-500 Volt 120 mA type.
The identification is etched into the dome of the balloon envelope.
This face on view was shot with dark field illumination.
The balloon envelope is 57 mm in diameter and excluding the B4 base pins, is 117 mm tall.
References: Datasheet & 1043. Type UU120/500 was first introduced in 1931.