We get used to looking at pictures like this and don't give it a second glance, because we think all of the cars were the same, but they weren't. The first 65 cars had a different boot floor that was hand formed and assembled from multiple pieces. Car number 10166 was first with the proper production stamping for the boot floor (photos above).

There is a significant difference in appearance of the early parts, having flatter surfaces, sharper bend corners and more weld joints
On 9/25/2023, John Prewer wrote:
"I have just bought 10169 and it has the early boot floor". (photos below).

I need to ask him for the body number. Possibly some overlap at the change-over point.