The 20D1 is a double signal diode. Possible uses would be demodulation of signal and AGC voltages. The 200 mA heater requirement suggests universal mains sets.

The two additional pictures show other views of the electrodes. The anodes are mainly flat with a very small circular cavity between the plates for the heater.

The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 47 mm tall.

References: Datasheet & ¹⁰⁴⁰. Type 20D1 was first introduced^¶ in 1949.

 
Pin Connections

B7G	1	2	3	4	5	6	7
	k(1)	a(2)	h	h	k(2)	s	a(1)

 
Absolute Maximum Operating Conditions^¶

Diode	Vh	Ah	Va	mAa
	9.5	0.2	175	9

Thanks to Frank Philipse for supplying the above PDF datasheet