There is of course no such date recorded in factory records.
The only choices for JDHT to use are:
“date chassis completed” this doesn’t mean car was finished
“invoice date” once again doesn’t necessarily mean car is finished but usually is.
“Dispatch date” OK this does mean car is finished ready to drive away and this can be before or after invoice date.
“Date Sold”
I will dig out a heritage certificate and compare it to a specific car in factory records to confirm which of these dates they are usinng