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

Answer this question, if possible (if impossible, reply "unanswerable"): What is the current musicologist reference for Chopin's pieces?
the Kobylańska Catalogue