When your registration was up around 2001 you’d get a “replacement plate” sticker to put on the upper right of the plate. Then when you paid the registration renewal the new plates came, it had the new expiration sticker and plates in an envelope. Then you mailed the old ones back in the envelope the new ones came in. But, at the time many vehicles didn’t have 2 plates, maybe some had theirs in a garage and never sent both back. I know the car I had the front plate got mangled in a fender bender so I only sent one back.