What Mike ^ said. Just for giggles I did a search on eBay for "1951 Missouri truck license plate" and didn't find a coresponding plate. The colors were all over the place.
I did find an "Antique" 1992 MO plate, however.

There ought to be a law against that. That would make me almost a triple antique, I guess.

On edit: here's a wikipedia article that should help you out.
Missouri Plates