The W521 set is exactly the same as the W519 Type 1 set, with the same numbering and players, except that the cards are mirror images of each other (e.g., reversed images). You can usually tell if a card is the W519 Type 1 if the shirt buttons to the left, and in the W521, the shirt buttons to the right. The W519 Type 1 set is difficult to find, but the W521 is tougher. There are four sets that are very similar:
W519 Unnumbered
W519 Type 1
W519 Type 2
W521
In order of scarcity (IMHO):
W519 Type 2
W521
W519 Type 1
W519 Unnumbered
with the Type 2 being the toughest, and Unnumbered set being relatively plentiful.
There are a couple of links here:
Link 1,
Link 2