Hi, all. I bought it, indeed as a curiosity. The seller had a "best offer" button on the listing, so I put in an offer and he accepted! I can confirm it has indeed been modified from an 1859 specimen. There is no w.w. and under extreme magnification one can see where material has been moved from the centre of the 9 to plug the "gap" at about 8 o'clock in the original digit so it looks like a continuous curve. But, my word, it has been done skilfully! I bought it as I remembered seeing one before (presumably Gary's above) and wondered how many others might be out there. I will try to take close-ups of the date later and post.
The seller also told me that he bought it from Seaby's when they closed down and said it had been in old man Seaby's (would that be Peter Seaby??) personal collection as a curio. Wonder if he bought it from the auction Bramah mentioned? No way of knowing that, I guess...
So, not a unique rarity - never thought it was - but a brilliantly executed alteration. Wonder why anyone would take the time and effort to do it, though?