...as long as the book is numbered in red on the colophon page at the back. this book was published very shortly after Buk's death. the signed sheets were tipped (glued) into the edition. There are no copies of this book signed on the title page, for example. Can you send a photo of the colophon page and we can say that it is 100% sure. If there is no colophon page then there could be a huge problem (as that would mean that this was a later printing...)
That being said, the signature looks right.