Thanks again Amran.

That one's close to the serials for the Belgian AF Congo watches, making it quite an early one.

The serial range for 7753s is more or less contiguous, with the occasional 7754 Skipperera amongst them (where the case is marked 7753 anyway). There is an occasional 3647 dotted amongst them - using up stock of Vj 92s perhaps. That would seem to indicate that serial and ref numbers were engraved at Heuer prior to assembly rather than at the case manufacturers, if they are able to readily switch back to a superceded reference.