Reuse on reggies
Most manufacturers in the US have a "fleetwide" N-number that may be applied to all their aircraft involved in pre-delivery testing. This number is applied temporarily and is usually NOT used for ATC purposes - instead the Mfr's name and test number is usually used.
Owners / operators often re-use numbers too; after removing a desired number from one plane they can re-apply it to another ship. It's been done a thousand times or more.