Bukfan: The two framed ones I have are dated '63, and as per Scott Harrison they were signed to be later bound into future books by the Webbs, which then never happened. If you look at the full photo of the thing you can see that the paper is book page size, so it seems very plausible an explanation. Scott got them directly from John Martin, so the provenance is rock solid, but even he has never seen this latest one.
Just thought I'd clear this up. :)