Unfortunately, the one Bill Piggott book I have makes no mention of this. Oddly, neither does my factory Spare Parts Catalogue (4th edition, I think, and showing various TS60001 and future changes from my 3rd edition copy, which doesn't show those). And I do recall in the past seeing at least one pre-60000 car, appearing to be original, with no blanking plate (but also with the other post-60000 features, so who knows WHAT the story on that car was?).
The mystery continues....