The CV1268 is a full wave rectifier based on the U50. The old service code is NU20.
The KB/Z identifies this valve as being made to specification K1001 or K1006, the B denotes qualification by a UK authority and the Z identifies the maker as MO Valve Company, Hammersmith.
The Maker reference and the twin rectifier sections.
The U50 also appears on the envelope indicating complete equivalence.
The oxide coated ribbon filament - note the welds to the supports.
The top of the filaments pass over a bridge in the top mica. Any heat related extension of the tape would be easily contained in the anode and thus no tension spring is fitted.
The classic envelope is 45 mm in diameter and, excluding the IO base pins, is 103 mm tall.
