Hello Dannybuck,
The 9D engine was made before, during and after WWII. This means you can find 9D’s identified with either the pre-war letter system; AA, AAA or AAA***A as well as the three number system used after the war.
Your 640/ is correct for a Villiers 9D made between 1946-49. They stopped making the 9D in 1949.
We cannot be more accurate than that. Most of the Villiers information from 1946-49 is lost.
At this time Villiers sold 9D engines to Eysink, Hulsmann, Sparta in Holland and Socovel in Belgium so your engine may have come from one of them.
Good luck,
Bob