The HL41 is an externally screened mains operated amplifier triode of medium impedance. It was designed as an oscillator or RCC amplifier.

It is not clear what frequency range this device was aimed at. A guess could be the first audio stage in a radio receiver. At the time Mazda had a passion for applying metalisation to valves even if it was not indicated electrically.

The classic envelope is 31 mm in diameter and, excluding the MO base pins, is 78 mm tall.

References: Datasheet, ¹⁰⁴⁰ & ¹⁰⁴³. Type HL41 was first introduced^¶ in 1938.

 
Pin Connections

MO	1	2	3	4	5	6	7	8
	h	k	a	-	g1	m	-	h

 
Absolute Maximum Operating Conditions^¶

Triode	Vh	Ah	Va	Vg	mAa	ra	gm
	4.0	0.65	250	-4.5	7.0	11,500	3.1

Thanks to Frank Philipse for supplying the above PDF datasheet