The CV1989 is the commercial SD6 and 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.
There is a circle printed on the glass with 805V inside.
The Cossor logo and name together with the commercial Type designation of SD6. The valve was made in Great Britain.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 45 mm tall.