Grizzly73,
You register the firearm without a serial number. There is a check box for such occurrence on the registration form. You then receive from the
Firearms Centre a certificate without a serial number, which makes it more useless than usual. Later, much later, you will receive a Firearms Identification Number sticker in the mail that you are to attach to the receiver of the firearm. These stickers are only issued for those firearms without serial numbers.
Don