The CV1989 (commercially the SD6) is a single diode. The anode for a second diode is included in the envelope but no heater/cathode is present. The capacitance is small suggesting a signal diode role, possibly as a vision detector in VHF televisions.
The Cossor name, Made in Great Britain and the SD6 Type designation.
The unused second anode that just balances the mica discs. The centre screen is connected.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 45 mm tall.
