The M8100 is Mullard's special quality version of the EF95. It is an RF pentode for VHF and above. The build is rugged to allow the valve to operate in conditions where shocks and vibration are present.
The anode is 9 mm square and 6 mm deep. On the 6mm face is a circular hole revealing the electrode structure.
The g2 helix has 4.5 turns to each single gentle turn of the g3 helix. G3 is internally connected to the cathode. The heater can be seen to be a coil of fine wire, covered in insulation, that is folded into a hairpin and gently twisted.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 37 mm tall.
References: Datasheet & 1040. Type M8100 was first introduced in 1960.