This is the famous piece better known as Nocturne op. posth. in C sharp minor. I play the only autograph version as published by Henle. It is an early version where there is a part with 3/4 bars at RH superposed with 4/4 bars at LH. It is not the most usual one. A candidate at the Tchaikovsky piano competition (Benjamin Moser) played this version a few days ago. It is also the one played by Claudio Arrau in his integral of the nocturnes for Philips. Of course, my interpretation is at a much more modest level, especially in this delicate part at 3/4 & 4/4. Chopin - Nocturne in C-sharp minor, Op. Posth.