The HL41 is an externally screened mains operated amplifier triode of medium impedance. It was designed as an oscillator or RCC amplifier. At the time Mazda had a passion for applying metalisation to valves even if it was not indicated electrically.
This device was aimed at being the first audio stage in a TRF radio receiver or a superhet with external signal detection. The HL41/DD added a pair of signal diodes to the triode amplifier.
The reverse is plain zinc paint.
The classic envelope is 33 mm in diameter and, excluding the MO base pins, is 79 mm tall.
References: Datasheet, 1040 & 1043. Type HL41 was first introduced in 1938.