Thanks to everyone for the atention.
My first doubt about of the Shadow Comet's existence came when I saw the right side painel of the machine in question, a Comet 1951. And the Vincent name in the rectangular marking is still in black, exactly as the Black Shadow that I'm restoring is as it was when I received it, since most of these machines had its black paint removed.
A friend of mine visited London last year and gave me a good magazine (The Classic Motor Cycle - number 11, november 2012), with the Big Red Vincent on the cover page. On the page number 27 there is an advertisement of the Bonhams (The Autumn Stafford Sale) you can see a picture of the unknown "Shadow Comet" for sale. The magazine was printed in England, what can I do? Belive it or not???