Perhaps you are right, but if your speculation is true, how would Seaport Traders ever know which shipments were paid and which were not? Klein's also made deposits of lumped sums, but they at least kept a list of the individual items. Do you have anything to suggest that Seaport Traders wouldn't have a similar system?
The rifle was purchased via money order. Money orders are handled more or less like checks, so they inherently leave an easy-to-follow trail through the banking system as they pass from institution to institution. Cash doesn't leave such fingerprints; it just gets piled up and deposited, no more questions asked. That's why it's the preferred method of payment for shifty business.
Anyway, If you read Michaelis' testimony, you'll find that he fingered his exhibit five as the notification from REA that REA had been paid. If you look at the far left of the doc carefully, you'll notice two things. The first is a field for "amount to be paid." It includes the helpful instruction, "for destination agent's use only;" that is, it's for the guys at REA's Dallas office. In fact, it's the spot where the agent on the receiving end tells his coworkers how much to remit back to the shipper. And just below that field, under "C.O.D. draft issued" there is a serial number stamped, somewhat sloppily. A draft is a form of payment, and REA is issuing it. The only people they would be issuing to in this case is Seaport Traders.
The draft leaves open an interesting question, since a draft can be a negotiable instrument like a check. However, Seaport Trader's probably did enough business with REA that they had an active account. I'm willing to bet that the draft was simply credited to ST's REA account, and whatever money was paid to REA was the balance of the account after it had been settled for the week/fortnight/month.
That's all kind of long winded. Here's the upshot:
1.) What you say you are looking for is Michaelis exhibit 5
2.) While there is the possibility that a draft was physically issued for the transaction, there is no good reason to
expect that it happened that way, and good reason not to.
At this point what more do you need?
How do you know that the revolver taken from Oswald at the Texas Theater is the one shipped to "Hidell"? I know that the revolver now in evidence is the one sold by Seaport Traders, but I don't know if that was the revolver they took from Oswald.
I have this funny feeling that you won't be satisfied until you locate a time machine, travel back to 1963, go to the Texas Theatre and actually see it for yourself. Just a word of advice: make sure you buy a ticket before you go in. They're sticklers for that in Oak Cliff. Even then, I kinda expect you won't be satisfied.
What I really want to know is what I have been asking for all along; conclusive evidence of a transfer from Railway Express to Seaport Trading for the C.O.D. amount they had collected.
As we are talking about events that took places in one fiscal year, I would expect their records to be complete.
A fiscal year doesn't necessarily start on Jan 1, and there's no guarantee how long that REA would keep the docs for a single transaction for long after everything is settled at FYE.