The stamp featured on the cards is a stylised "W", which represents part of the original Wizards of the Coast logo.
Similar to the original run of Prerelease cards, Wizards only produced a small amount of these (7 cards from 6 expansions). The majority of them were affiliated with Pokémon-related magazines. Foil stamped cards would not make an appearance again until the Best of Game promotional cards. It is possible that the "W" may have been the inspiration to stamp the special versions of these cards with "winner".