The CV131 is a general purpose amplifying pentode. The commercial equivalent is the EF92 and the M8161 is a special quality version.
The KB/D identifies this valve as being made to specification K1001 or K1006, the B denotes qualification by a UK authority and the D identifies the maker as Mullard Radio Valve Co., Mitcham.
Seen in close-up the inner arrangement of the grids is visible where the anode shape allows.
The screen below the lower mica shields the anode lead. The screen itself is connected to g3.
The thin glass tube envelope is 17 mm in diameter, and excluding the B7G base pins, is 46 mm tall.
