I see a metal rivet on the bottom plate, so there might be more rivets on other pieces. The only sure way to open rivets is to cut or drill them.
Try pulling the feet off the chassis, manufacturers like hiding important screws under rubber pads. There might be a screw hidden out of sight (and perhaps also out of reach) inside a drive bay or any other user-openable compartment. Basically the last place they think you'd think to look is where they'll put that last nasty little screw that locks everything else down.
You might have missed screws hidden under any sort of sticker or label.
Someone could have used an adhesive to the side panels. So that opening them requires some force and leaves some permanent indication that unit has been opened outside of their service center. (Old computers can even get glued shut by years of nasty gunk in their operating environment.)
If all else fails then resort to Man Tools - crowbar, sledgehammer, axe, shovel, chainsaw - they'll open anything.
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[/Korth]
