The D43 is an indirectly heated television signal demodulator diode. The D42 is also a TV diode and dates to 1936 and is very similar. The D42 was preceded by the D41. The D41 has less emission and the heater is rated for 4.0 Volts at 0.3 Amp..
The Marconi brand label.
Thanks to Mark from Vintage-parts for sending these images.
The classic envelope is mm in diameter, and excluding the B4 base pins is mm tall.
Absolute Maximum Operating Conditions¶
Updated February 27, 2017. shape:classic construction:pinch type:diode age:1930.1940 base:b4 heater:4.0v pins:4 pin:1.a pin:2.k pin:3.h pin:4.h