We have used these boxes for many many years for storing financial records-personal as well as business. They have held up in the heat of the attic and served their purpose for preservation.
Some of the boxes are not square. Look at the top corner where the cardboard is joined and glued. Check to see if the top edge lines up. Lot I purchased were as much as a half inch offset from each other. This will cause the top cover not to sit on an even flat surface. This might cause a stack to lean if heavy boxes are placed on top of each other.