Show me where Klein's had the serial number C2766 on the 40.2" rifle and shipped it to PO Box 2915? Show documentation with 40.2" and it's associated catalogue number of C20-750 with the serial number C2766.
No Other, if you're going to continue on this line of argumentation, it would be well that you realize the burden of proof that you alone bear, and how weak your current position is. The WC established the null hypothesis that we all must work with: that is, the WC gathered testimony and evidence that showed, at a minimum, that Crescent Firearms of NY shipped to Klein's of Chicago a MC rifle bearing the serial number C2766; that Klein's shipped a MC rifle of the identical serial number to one "A. Hidell" of Dallas, shown to be LHO. The Italian Armed Forces Intelligence also confirmed the serial number C2766 for a MC rifle. (WCR 118-19). A rifle with that serial number was found in the TSBD after the President's murder, bearing LHO's latent palm print. And that's not all the evidence that links LHO to C2766.
Now, against all that--the null hypothesis--you seem to want to establish an alternative hypothesis, though I'm not sure what it is.
You are requiring your critics to explain an anomaly which does not overturn the null hypothesis, and thus shifting the burden of proof. The anomaly is not strong enough to destroy the null hypothesis. The proof you demand that Klein's substituted a 40-inch rifle for the 36-inch rifle apparently does not exist. Does this single anomaly destroy the null hypothesis, in your mind? If so, I think you are exhibiting a poor ability to weigh evidence in context.
There is so much evidence that C2766 was ordered, owned, and fired by LHO that the apparent anomaly can reasonably be resolved by the circumstantial inference that Klein's simply shipped a 40-inch MC rifle in response to a coupon filled out for a 36-inch MC rifle. The shipping order gave a "100% Money Back Guarantee," but it can reasonably be inferred that LHO, if he noticed the 40-inch length at all, did not return the rifle for a refund.
Your demand for all-anomaly-resolving paperwork strikes me as of a piece with those CTists who say that unless there is a photograph of LHO in the Cuban and Soviet embassies in Mexico City, it is incredible that LHO was in Mexico City, because photos were often or typically taken inside the embassies.
Your own ideal of an adversarial trial would have permitted defense counsel to raise the seeming anomaly of the rifle length. I strongly doubt that that one point would have led a jury to reject all the other evidence of LHO's purchase, ownership, and use of C2766.