The 500 apart thing may only relate to Aussie made shells. Deluxe, Cooper S and GT did it.
Early Coopers had body and chassis numbers the same or only a few apart. The register shows this.
Early 850's also have a number under the ID plate that is very close to the chassis number. Here are a few examples.
Attachment:
18620 and 18688.jpg
Attachment:
19043 and 19051 .jpg
Attachment:
22014 and 22085.jpg
Sorry I can't answer your question but I always wondered why some 850's they had a hidden number under the ID plate only a few dozen numbers away from the chassis number. It may be that early 850's didn't have the body number on the Rad shroud and the number under the ID plate is the body number.
If this the case then as production went on the few dozen numbers apart would turn into a few hundred apart by the time your car was made.