Had a defective lock button on my iPhone X. None of the Apple Stores in my area had any in stock, so I ended up waiting 2 days for a "refurbished" unit to be shipped to the store. It has an "N" serial number. Yes, I would have preferred to exchange it for another retail unit, but that was not an option (I could have returned it for a refund and then tried to find another iPhone X, but felt the "N" vs "M" issue wasn't worth it).