Impossible standard?? Simply because some person swears that he's been to the moon and has a rock from the surface of the moon to prove it .... Does not make his tale true. Simply because Mr Kaiser said that the clip board was his, and Lee Oswald had used it does not constitute proof that Lee had been using it on 11/22/63....
It does, however, provide a very good basis for reaching that conclusion. Also factor in that no one else who had access to the floor (co-workers) ever came forward and said that they had used that clipboard and placed it there. You can believe anything that you want, but the totality of facts, common sense, and evidence lends itself to Oswald using this clipboard on 11.22 and leaving in that location. Also keep in mind that it had invoices dated 11.22 on it. Kaiser was not at work that day. So someone used that clipboard on 11.22 to fill book orders. That narrows the list down considerably. And Oswald was in the building and known to use that clipboard. It allows us to make a reasonable inference.