I used to visit the factory early 1970s & believe/confirm that the despatch date was the manufacturing date as bikes were shipped to get payment asap, not held in stock. It was motor industry practice for stock to be held at dealers, not at the factory. The practice of filling acres of tarmac with unregistered cars until dealers call them off stock is fairly recent and certainly didn't happen with BSA and still doesn't happen with bikes.
What's more, there wasn't the computerised standardisation that there is now. If a customer wanted something different for their bike, the dealer could either swap bits before release, or order the bike with those bits from the factory.
The problem we have today is that most of the population, including some owners (not just DVLA) do not know how factories worked in those days.