The KT66 is a very well known audio output beam tetrode valve widely regarded as the best audio valve ever designed. Here we have a VT75 that is listed as an equivalent but that looks much like a CV1947 that is equivalent to the 6L6G that the KT66 was developed from.
The UK military up arrow on the glass. At the base of the anode can be seen the bright beam forming plates, the flat fronted cathode tube and where the leads from the pinch were spot welded to the main electrode assembly.
The control grid has a box heat sink at the top.
This slightly enhanced image shows the VT75 near the bottom as well as the oxide coated cathode under the top mica. The fingers of the top mica are bent down from the tight fit into the top of the envelope.
The classic envelope is 56 mm in diameter and, excluding the IO base pins, is 122 mm tall.
References: Data-sheet, 1004, 1005 & 1040. Type VT75 was first introduced in 1939. See also1939 adverts.