northman wrote:

So Merlin Unwin was kind enough to answer my enquiry about the sheets. He informed me that he didnt start working at George Allen and Unwin until a couple of years after the publishing of the silmarillion so unfortunately he had not seen these exact sheets, but he did know that such signatures were sent from GAU to the american publisher before printings of the US edition.

Then Uruloke mentioned that some publishing veterans participated in the 'BBC mock up thread' on Brian Sibleys FB. I sent them photographs of the sheets and one of them, David John Fielder, could inform me that these sheets were indeed the real thing and was a copy of the first Silmarillion printing sent to the american publisher to prepare their printing. Fielder used the phrase 'pretty rare' defending the upcoming cost of having @Duneidan making a nice clamshell for them :)

Thank you to Khamul and Uruloke for helping me sort out a bit about the mystery of these sheets. I feel very lucky now that I picked them up.