
That is a killer find

As long as it weighs the correct 8.1g, then you should have a nice valuable error on your hands. Your technical understanding is correct, an unstruck planchet has a smaller diameter than a struck coin. The
Presidential dollars are struck and then have the edge lettering applied in a secondary process. The coins are force spun against a bar die so the tolerances have to be tight. If a smaller planchet were to get edge lettering, it is to be expected that the lettering would be weaker than that of a normally struck full sized coin. However, there have also been normally struck Pres Dollars with weak edge lettering as well.
In 2007, PCGS was offering a $2500 reward for the first golden dollar planchet with edge lettering. The reward was collected in the same year but not many of these errors have turned up since then, I could see this being a $500+ error if you were to get it certified. Even if the coin did not have any edge lettering present, it would still be a desirable error. Golden dollar planchets are worth around $75.