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 envelope seems impossibly large for a single diode but during the later 1930s working successfully at around 45 MHz was a difficult task. The future lay with the miniature diode such as the D1 and the IO based 6H6. Post war the B7G based EB91 style was employed until the semiconductor diode took over all such tasks.
The Osram paper label.
Side view of the anode.
A closer view of the diode.
The classic envelope is 41 mm in diameter, and excluding the B4 base pins is 101 mm tall.