Numbers on the outside
Not a good idea if it's a leased machine.
Those numbers (the key) can only be used on the orginal system so it's unlikely anyone can "borrow" them.
Also if it's a leased machine and you remove the COA (or move it to the inside), you've just removed about $100 from the value of the machine and you will probably get charged for the missing COA when you return the unit.
Jeff DeWitt