Well, in the Arthurian mythos, Percival did find the Grail; so the definition might better be something that is "unattainable except by the greatest sacrifice".CarlosV wrote:Holy grail is the unattainable. Once you find it, it is not the holy grail anymore. Therefore, the Buddy Bolden and the Mark Twain cylinders, de Reszke discs fit in my definition, until you find them and demote them to simple rarities ...
That said, I agree that your examples are good. In the machine category, we might include the original Tinfoil Disc Phonograph, the Columbia MG, and perhaps the Diamond Disc prototypes...