The VT79 is a beam tetrode equivalent to the commercial KT8 and fitted with a ceramic base cap. The upper frequency limit is 65 MHz. In 1941 with the advent of the CV register that unified the UK service procurement of valves the RAF VT79 became the Common Valve CV1079.
The M-OV KT8 was based on the American 807 and was not as good.
The main image shows both the VT79 and KT8 Type designations.
The KT8 in the typical M-OV ellipse. The screen under the lower mica is present as are ceramic supports for the box anode.
The anode with side flanges for better heat transport away from the centre of the working face.
The top mica and screen above.
The classic envelope is 49 mm in diameter, and excluding the B5 base pins is 114 mm tall.