Mullard PM3 triode, measuring 100x45mm..

The label round the base reads "This valve has been carefully tested and no claims for replacement will be entertained for valves used under conditions other than those specified below
MAX. FIL. VOLTS 4.0v.
FIL. CT. 0.075 amp.
MAX. ANODE VOLTAGE 125v.
THERE IS PRACTICALLY NO GLOW FROM THIS FILAMENT"