The Mozart Catalogue:
Mozart's catalogue was most famously compiled by Ludwig von Köchel (1800-1877). It was the first complete catalogue of all of Mozart's completed works to date, starting at K 1 and ending at K 626. Köchel's complete 551 page catalogue Chronologisch-thematisches Verzeichnis sämmtlicher Tonwerke W. A. Mozart's (Chronological-thematic Catalogue of the Complete Musical Works of W. A. Mozart was published in 1862 ref1. Several attempts at compiling Mozart's works were made prior to Köchel, but none of them were fully achieved. The numbers following the titles of Mozart's works refer to this catalogue.
But here's where it gets even more complicated:
The Köchel catalogue was updated and nearly completely revised in 1937 by Alfred Einstein, a musicologist and possible 6th cousin to famous theoretical physicist, Albert Einstein. Mozart's catalogue needed to be revised because since the original Köchel catalogue, dozens of other works by Mozart became known, which were not included in his catalogue. Thus, several of the original Köchel numbers were changed to reflect a chronological listing of Mozart's works. Most of these changes are reflected in additional numbers following the original Köchel numbers. For example: Serenade No. 3 in D major for Orchestra, K 185/167a "Andretter." The K 185 is the original Köchel number and the K 167 is Einstein's revision, placing it in the correct chronology in light of the discovery of new works. However, since Einstein's catalogue, other Mozart works have been discovered. As such, rather than completely revising the catalogue once again, modern cataloguers have simply added letters (a, b, c, d, etc.) to these new discoveries and the older ones in order to place them in the correct chronology. Thus, this Serenade is probably the first piece under K 187/167 as indicated by the "a." Several other pieces are also part of that particular Köchel number.
With that said, I find a useful tool to be the Neue Mozart-Ausgabe: Digitized Version, where you will find Mozart's most up-to-date complete catalogue with the updated Köchel numbers by Einstein and more recent cataloguers, musical scores, audio excerpts from selected pieces and general information concerning the structure of Mozart's pieces. It's the best source on the internet for Mozart's complete works, but use with caution. Don't copy and publish anything without their consent. It's intended only for personal use.
To be continued....