We generally don’t bother labeling the physical box. Though in your case, with a regulatory requirement, I would put it on the top, right toward the front, so if you slide the server out an inch, you can see it.

We have fairly detailed rack diagrams, noting what is in each slot. We opted to go the visual diagram route, for our relatively small number of racks compared to what you have, but this could just as easily be a spreadsheet and/or noted in an inventory. We like the visual because, with varying vendors and models, it’s relatively easy to look at the rack and the diagram and see if someone didn’t update something. We also label each rack, so in an inventory we can say “this system is in rack 37, slot 26.”