The Date on 320691 was read directly from what I believe to be the original Certification Label. In my survey page I list all the data from the AN dispatch record for 320691 which includes the frame number (stamped into the headstock) so I know the frame and engine left the factory together. This bike and a few others are why I believe that at least in 74, the Certification Label was stamped and on the frame before they started building the bike. It came off the assembly line (Date off track) Oct 23, 1974 and was shipped (despatched) Nov 26, 1974. If oyu keep looking you've find higher serial numbers shipped earlier according to the factor records.
"Built" is a hard thing to track down and I believe that Norton changed the way they did things a few times between 71 and 75. Also, it appears that some bikes failed testing and were eventually finished later.
The more people who provide info, the better our understanding can be:
https://www.gregmarsh.com/MC/Norton/CommandoSurvey.aspx