The CV281 is also the commercial X61M triode hexode. It was purpose designed as a radio frequency changer.
The input signal would be coupled to the frequency changer via the top cap connection to grid 1. The 6K8 is an equivalent and the data for it shows the top cap connected to g3. In both cases the input signal is connected to the top cap irrespective of the internal construction. It also suggests that this valve has separate triode and hexode sections, whereas the 6K8 employs a clever arrangement of grids.
There is a paper label attached to the top of this exhibit. Note the wire connecting the zinc paint to the base pin.
The direct equivalence of the CV281 and X61M is confirmed by the envelope.
The label dates this valve to 1966. The place of issue is Donnington.
The classic envelope is 38 mm in diameter and excluding the IO base pins, is 100 mm tall.