The CV1263 is a half wave mercury vapour rectifier and the prototype was the RG1-125. The old Admiralty service code is NU8 which is also present on the envelope.
The twin corrugated filaments.
A closer view of the filaments.
The anode is a wide open cylinder closed at the top.
The classic envelope is 50 mm in diameter, and including the ES base is 140 mm tall.
