Answering my own question, an Aeromax friend of mine of this parish pointed out the obvious, that the Aero Coupe would likely have its own series of Chassis numbers which could make mine the 39th or 40th one built out of 40 (assuming they started at 4000). As mine was very late this could make sense. Mine also has the paddle shift option introduced with the Series 5 Aero 8.

Don't think you can apply logic to the numbering of chassis with Morgan! :-)