Anther problem you have is not all of the Cheerios dollars had the prototype reverse. What percentage had the prototype reverse I don't know. But for the one that don't, once they are removed from the Cheerios packaging there is no way to differentiate them from any of the other 2000 P sac dollars.
If they aren't savvy, they may have spent them, or they may have just tossed the package in a drawer somewhere. After all, to them it's no big deal, just a dollar.
Make that 100%. The prototype reverse wasn't identified until a few years later.