The correct answer to your question (coming from gift tax law) is that this type of gift is what is called an 'uncompleted' gift. True the Greens gave Charles a gift letter, but without physically transferring the asset to Charles, the gift remained to be completed. Because the gift is uncompleted, it is revocable.

The Greens can change their minds up to the time they die, after which they can no longer change their minds. At date of death, the gift is complete.

If they intend the local collector's club to have the coin collection at death, they need to revoke the gift letter to Charles and execute a new one to the club.