Why do some musicians choose a name that is a letter followed by a dash and some numbers (usually a 3 digit number)? I have seen this before a few times, but never found out how or why this tradition started. The two musicians I can think of right now that have this naming style are F-777 and K-391, both of whom make electronic music.
The composer didn't choose "names" like this for his/her pieces. Some composers didn't bother to make a list of everything they had written during their lifetime, and somebody else had to make a catalog of their works after they had died.
The "letter" is often the initial of the person who made the list and assigned the numbers to each piece. For example Mozart's music was cataloged by Ludwig von Köchel who published the first version of his "complete" list of Mozart's music in 1862, about 70 years after Mozart's death.
His "K numbers" are still used to unambiguously identify each piece, though his original list of numbers is still being extended as more works by Mozart are discovered, and some pieces have been renumbered. For example the first Horn concerto was originally K. 412, then renumbered as K.514 and in the latest list is K.386b! The latest revision of his list, made after his death, are known as KV numbers (Köchel Verzeichnis. i.e. Köchel Catalogue.)
Other composers whose works were catalogued in the same way include Schubert (D numbers, compiled by Otto Deutsch - more than 120 years after Schubert's death!), Bach (BWV numbers, the BWV standing for Bach-Werke-Verzeichnis, i.e. Bachs-works-catalogue), Frescobaldi (F numbers) and Haydn (H numbers, compiled by Anthony van Hoboken, who incidentally had the same initial as the composer)
Some composers' works have been cataloged (inconsistently) by several musicologists. For example Scarlatti's sonatas are identified by L numbers (Allesandro Longo), K numbers (Ralph Kirkpatrick - these K numbers have nothing to do with the Mozart K numbers), P numbers (Pestelli), and S numbers (Synzyusya) - and to add to the confusion, Longo published a supplement to his original list of L numbers, also using S (for "supplement") numbers.
Handel's works have also been cataloged at least three times, using HG numbers (Händel-Gesellschaft), HHA numbers (Hallische Händel-Ausgabe) and HWV numbers (Händel-Werke-Verzeichnis, a similar name to "BWV").
Some of these lists attempted to number the composer's works in chronological order - this explains the renumbering of Mozart's works, as more facts about his life were discovered. Other lists follow different systems. For example the BWV numbers group Bach's works by "genre", starting with all of his church cantatas (BWV 1 through to about BWV 200) irrespective of when they were written.