In retelling the story, I forgot that I'd received this in parallel with the new e-ticket.
It appears to be an invoice for collection of €70 - the fee that I had been quoted several times by the KL website, and which I would have been happy to pay - and despite the reference to my credit card number near the bottom (blacked out) I instead expected that I may have to pay this at check-in.
The shock of seeing a charge of €260.01, instead, caused me to forget about this invoice. Perhaps, on the strength of this invoice, I could have "forced" them to keep me on the changed flight for the stated €70 rather than the charged €260.01, but that never occurred to me.
And yes, despite the mails emanating from Amadeus, this booking was originally made directly with Air France on their website.