The ME1403 is a subminiature electrometer pentode. Mullard describe the the ME1403 as having a grid current of 3 x 10 to the power -15A.
To avoid drift the filament must be heated before anode volts are applied.
Such electrometer pentodes would be used for pH meters and other critical measuring devices.
The connections are taken clockwise when looking at the underside of the base. Pin 5 is at the centre. The red spot is slightly clockwise from the anode. The base is designated B5J/F by Mullard.
Seen from the side the wire grids become clearly visible, with the anode showing as two strips at the top and bottom of the picture.
