Here in California, that sort of notice used to mean that it didn't originally (nor now) meet smog requirements. If you cannot get a license plate for it, in Calif. you can't get insurance for it either.

If I remember correctly, a tag number of 420 was OK to license anyplace. One with a 421 tag was OK in 49 states. I dunno what if you had one of the 422 tags.

Wm.