Can you prove that Oswald owned that rifle? Or that that specific rifle is the one in the backyard photos? That's LN logic. Make up conclusions that you can't prove and state them as facts. It doesn't have to make sense, it just has to convict Oswald.
It is really not essential to the case that Oswald owned the gun. If the ownership of gun found on the 6th floor could not be tied to Oswald (ie. suppose Klein's had a fire and lost all its records), the case would not fall apart. In any event, one does not have to prove all facts separately beyond a reasonable doubt in order to prove guilt beyond a reasonable doubt.
Having said that, proof" is accomplished by persuading a trier of fact that the evidence meets the standard of proof. That's all. The evidence may not persuade you. But I would venture to say that if you picked any jury and asked them, based on the evidence, whether it had been proven that Oswald owned the gun, they would find that it had.