I just installed two of these from Ebay. They are factory.
If you look closely you will see that there are 4 bolts (two on each side) holding the box to the frame. There are also two safety cables to avoid the battery dropping to the floor.
Of the 4 total bolts, 3 of them have slotted holes corresponding to the respective holes in the frame of the box. That leaves one bolt without a slot.
To remove the boxes, the non-slotted bolt needs to be removed entirely. The other three only need to be loosened so that you can slide the box so the slotted holes in the box frames can come off the loosened bolts.
Warning the battery weighs about 75 lbs and will drop when you get out of those slots. That is what the restraining cables are for.
After the battery box is hanging you can remove the top or remove the restraining cables and lower the whole box (using a jack or something else). I have not tried to put a battery in or out of a hanging box although it looks possible it might be wiser to lower the whole box.
I have a hydraulic table that I plan to lower both my two boxes in unison. There is a pretty long service cable from the factory that connects the aux battery to the Car battery. I did something different.
I would practice reinstalling the box while it is empty so you know what is required.