Hi kiwiringy,
You'll be happy to know your chances of precisely dating your engines are close to zero. Villiers give the range of years an engine was made but little information on precisely when an individual engine was produced. Not only that but if the order was big enough Villiers would make all sorts of minor alterations to an engine that look significant but have no real impact to the basic engine underneath. For example, they would change the position of the sparkplug, fit fixed ignition or adjustable depending on how they attached the magneto backplate. Fit coil ignition or magneto ignition with a dynamo for the lights, have a different shape to the inlet manifold, the minor permutations are endless but none of them affect the basic engine so a standard workshop manual applies to all.
In addition to the above in your own case they made a special drive side crankcase for the Cov Eagle to mate with the early enclosed primary chain. This makes a Cov Eagle engine very easy to identify.
As to the engine number prefix that identifies what engine you have no complete list exists. There are some very extensive ones in various Roy Bacon publications but he makes it very clear they are incomplete, that he was unable to obtain them all. As a result I have been slowly building a little list of engine numbers not in Bacon and have 50 odd so far.
On this list is AU as found on your Mk XVI-A engines. I'm fairly confident these were built for James. Certainly James sold well in prewar NZ, there are enough survivors for us to know that, so it looks like you have two James engines.
Cheers,
