Mystery Plate alpha attaches to a Pokémon in play, letting it borrow the card's attack in place of its own for the turn, after which the card is discarded. The four Mystery Plate cards, alpha, beta, gamma, and delta, share this identical structure, differing only in which of the four is named as the one to discard in their respective texts.
All four Mystery Plate cards were printed together in the WOTC era's Skyridge set, illustrated by Hiromichi Sugiyama. Our catalog lists 4 printings in 1 set.
Each Mystery Plate letter has its own Skyridge card number, #133 through #136, so alpha, beta, gamma, and delta are four separate cards even though their templated wording is nearly identical. Bulbapedia files Mystery Plate alpha among Trainer cards that can inflict Burned, Paralyzed, and Poisoned, and among those that move Trainer cards from the deck into hand.
Mystery Plate alpha is Skyridge #133, the first of the four
Mystery Plate beta, Skyridge #134, shares the same attach and discard structure
Mystery Plate delta is the fourth card in the four part set
Our catalog lists them only in the Skyridge set.
Each names itself as the card to discard at the end of the turn; the rest of the printed rules text is the same.
From Skyridge (2003) · 1 printing
Attach this card to 1 of your Pokémon in play. That Pokémon may use this card's attack instead of its own. At the end of your turn, discard Mystery Plate α.
From Skyridge (2003) · 1 printing
Attach this card to 1 of your Pokémon in play. That Pokémon may use this card's attack instead of its own. At the end of your turn, discard Mystery Plate β.
From Skyridge (2003) · 1 printing
Attach this card to 1 of your Pokémon in play. That Pokémon may use this card's attack instead of its own. At the end of your turn, discard Mystery Plate γ.
From Skyridge (2003) · 1 printing
Attach this card to 1 of your Pokémon in play. That Pokémon may use this card's attack instead of its own. At the end of your turn, discard Mystery Plate δ.

Mystery Plate
Classic Trainer (WOTC era)
First printed in Skyridge (2003)
Attach this card to 1 of your Pokémon in play. That Pokémon may use this card's attack instead of its own. At the end of your turn, discard Mystery Plate δ.