It is truly amazing that the contrarian brothers can attack the evidence of Oswald's guilt on the one hand as the potential product of fabrication, and then turn around without missing a beat to contend that they are not suggesting a conspiracy. If "PP" is circled on Klein's form indicating that they shipped a rifle via parcel post to Oswald's PO Box, the contrarians argue this doesn't prove they shipped the rifle. We are told that "anyone" could have circled PP or wrote the serial number of the rifle on the form. Who and why these mysterious people are doing this if not part of a conspiracy to frame Oswald? We will never know because that is where the contrarians go down the endless circular rabbit hole of lunacy. We are told they aren't claiming anything and have no burden of proof. Why? Because they recognize the absurdity of these alternative explanations having any validity. They don't want to defend them because they make no sense, lack even a scintilla of support, and are otherwise entirely baseless. It is defense attorney nonsense.
This evidence or material or information or data (whatever term one wants to use) exists. How did it come to be? How was it created? Who made it? It's either the creation of the company or, more accurately, people working for it or it was the creation of someone else. Some thing or some body made it. It's not in a quantum world of existing and not existing (although who knows, that may be their next idea).
So who made it? We say it was Kleins. They say? They won't answer. They can't answer, as you point out, because an alternate answer exposes the absurdity of their claims that it's not authentic, of their attempts to make it disappear. It exists. It's either authentic or faked. If it's not authentic then it's faked. By who? The honest conspiracists give us an answer: "the government". The not so honest conspiracists give us silence.