Welcome to FABO, and nice car too!! Now, on to your question...
In 1966, Plymouth would NEVER compromise the identity of a top trim model by putting a lower level trim item on a car for sale!! Model recognition was paramount in getting and keeping buyers, and as usual in advertizing, showing your top of the line stuff gets them in, and maybe generates a sale for that model, or hopefully at least a sale on a lower trim model!!
Now, I would agree with the above that this was replaced with a Valiant grille, maybe to replace the original which may have been in poor condition. They're hard to find in any condition, and back then these cars were out everyday in all kinds of weather.
At any rate, if it was my car I would be on the lookout for the best examples you can find and pay the price, you won't be sorry you did. And you need the center metal piece too, as it is cut out differently for the Cuda emblem as opposed to the Valiant "V"!! Those parts were just for sale here a few days ago!!
Welcome again and enjoy that car. As you can see, I'm a little biased to the 66's myself!! Geof
