This is taken from ETKA.
The dealer seems to be correct. The current part number is 1K2 423 055K. This replaced 1K2 423 055J last year.
1K2 423 051 CA is an old part number that was replaced by 1K2 423 055F (no date given). This part number still appears to be current and is shown as the steering rack for a Toledo.
055F is not listed for the Leon.