The NT20 is a directly heated triode. The NT indicates a UK Navy transmitting valve. With the advent of the CV system for Government procurement the Type was renamed the CV1208. The prototypes were the P625 and the PM256.
The front to back dimension is 6 mm.
The base pins and above the pinch is the lower mica. The oxide coated filament can be seen through the hole in the mica sheet.
The top mica and filament tension springs. The filament coating has flaked off in places.
The four filament strands and the grid winding.
The classic envelope is 45 mm in diameter, and excluding the B4 base pins is 103 mm tall.
Reference: Data-sheet. Type NT20 was first introduced in 1933.