The D3a is a special quality frame grid pentode for wideband amplification. The mutual conductance is very high at 35 mA/V.
Present day use (2014) is for audio voltage amplification.
The working parts are small in relation to the envelope. The anode is split into two plates, one either side of the central cathode. Beyond the central cathode and grids and part way to the anode are the beam plates. Being Mullard these are referred to as grid three.
The rectangular gap in the beam plates can be seen. The visible grid is the screen grid and the windings pass flat across the cathode.
In this image the screen grid is easily visible but there is no sign of an inner grid, just a grey sheet for the cathode.
Here the valve has been photographed with the Nikon DF and 60 mm macro lens. There is just a hint of the control grid.
Actual pixels from the DF. The depth of field is very small but on the left the grid winding is visible both on the support and part way across the face of the cathode.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 54 mm tall.
Reference: Datasheet. Type D3a was first introduced in 1958.