Don't take anything literal from the illustration. I have no idea how the Carcanos were placed in the box. May have been 2 rows of 5 or some other configuration.
I can't prove it to your satisfaction but I believe the Carcanos were shipped in cardboard boxes. This box, for example, could hold ten Carcanos and it claims to hold up to 80 lbs. ( Link ). Back in the 1960s, balled-up newspaper and bagged popcorn or wood chips made lightweight packing material. Maybe a few cardboard cut-outs that supported the rifle inside the box.
Can't see them using wood and all that extra shipping weight for surplus weapons with a small profit margin. Since the container was cardboard, a shipper might be willing to charge "Net Weight" only (weight of goods, excluding container) to secure a contract.
I have some doubt about the "net weight" theory, since shippers usually deduct "tare" from containers they provide to clients. (I'm willing to be corrected on that; I am no expert on shipping, and yes, I am not - unlike the Pope, allegedly - infallible.)
But if we go with your proposition on shipping weight of the rifles, 750lbs for 100 units, then the rifles would be 7.5lbs each, i.e. 40" rifle, not the 36" model Oswald ordered. No? Yes?