@pokemonrevolution recently presented a new and likely theory for these Grey Thick stamps, which I will quote here: Grey Stamp Theory Grey stamped 1st Edition Base set cards for the most part have commonly been considered to exist because of either A) The ink was low when they were stamped B) The stamp was not pressed down with enough force to create a nice dark color
Arthur is an incredibly powerful card that allows you to earn lore with your weak questers like Lilo, Making a Wish and Pinnochio, Star Attraction, then bounce them back to your hand to keep them safe from challenges - while also rewarding you with two lore for doing it
Considered by many to be overpowered cards when it comes to gameplay, that fact has not impacted their value or collectability
Much of what separates cEDH and traditional Commander is the access to fast mana, and game-winning cards that need few other cards
ready to join your collection
Note the double strike is just on your turn, not for defense