https://www.psacard.com/pop/baseball...williams/37470
Looks like only two panels have ever been graded by PSA (both only AUTHENTIC so your high grade one might get the same treatment). So most likely, the owner has not requested they be added to the Ted Williams Master Set. There is a requirement to add a card to a set that you have to be "working on the set", so you have to have created one with at least five items, I believe.
That way people can't just buy up a super rare item and add it to the registry unless they have some skin in the game.