The M8100 is Mullard's special quality version of the EF95 it is also labelled 5654 demonstrating their direct equivalence.
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 outer grids g3 and g2 are easily seen through the hole in the side of the anode. g1 is just visible close to the cathode.
We have another example with complete dual labelling.
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.