Works published since 1857 have received alternative catalogue designations instead of opus numbers. The present standard musicological reference for Chopin's works is <answer>the Kobylańska Catalogue</answer> (usually represented by the initials 'KK'), named for its compiler, the Polish musicologist Krystyna Kobylańska.
What is the current musicologist reference for Chopin's pieces?
the Kobylańska Catalogue