In this answer the tournament rules are quoted:
3.3 Authorized Cards
[...]
Players may use cards from the Alpha printing only if the deck is in opaque sleeves.
Why is this? As far as I remember there was an idea of having different card backs for each edition/expansion, but it was decided later to have the only one card back, which makes all cards compatible.
What makes cards from Alpha different so that they could be recognized without an opaque sleeve?