I'm not sure it's real. Cool photo. The cursive "s" is unusual for him, as are lower case letters in general. Usually, he wrote in all caps. But there are examples of him using that sort of an "s." Here's one; the Christmas card from him and Linda Lee has lower case letters with a curvy "s" in the word "Christmas": https://www.finebooksmagazine.com/f...rst-editions-original-art-and-letters-auction Bill Roberts would know.
I'm rather on the fence, but leaning toward genuine. Since it's not a full signature, it's difficult to say. Working against it is the script; as Rekrab notes, Buk usually wrote in all caps. But the "Buk" looks like it could be his hand. The "h" at the end of wish also bears resemblance to how the "h" in "Charles" often looks in his signatures.