Not a particularly common caliber.
Not a particularly common shipping container.
38 Super is very popular in Mexico because they cannot have military calibers like 45 or 9mm.
I bet someone from Mexico in the meat packing plant had it on them. And he decided he needed to dump it in a hurry. Say for example, Immigration or other cops walked onto the plant floor, etc.
Drop in in the box, close it, send it on down the line.
It’s a very popular caliber in Mexico. Mexican citizens are prohibited from owning “military” calibers such as .45ACP and 9MM.
Since 1911s have been chambered in .38 Super for decades and are not considered a “military caliber” (whatever that really means) the 1911 in .38 Super is very popular there.
My theory is someone got whacked in a Mexican slaughter house, and having seen too many American mafia movies they thought it would be funny to wrap the gun up with some meat for some peon to find.