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.
Someone may have put it there with the idea of shipping it to someone in Mexico.