The 56001 is a diode for operation at saturation. The function is for use in stabilising circuits.
The valves is labeled as Mullard but foreign made. The seal of the hard glass disc base to the softer envelope tube looks to be the of the typical Philips pattern. The locating pip is moulded into the glass and the pins can be seen to pass through the glass where the tops are used for mounting all the electrodes. Pin 4 and 5 are connected together.
The filament is a single vertical strand passing through the centre of the anode tube.
The top mica and the getter supporting ring fixed to an anode support.
The thin glass tube envelope is 20 mm in diameter, and excluding the B8A base pins is 69 mm tall.
Reference: Data-sheet. Type 56001 was first introduced in 1960.