L means Bishop, S means Knight and K means King. By the way, W. Speckmann (first problem December 1999) was his teacher. If you are interested, you may read the first page of the book, that he is planning to write now. If you want to cooperate with him on this interesting project, you can contact him, too. If you speak German, you can contact him in order to translate that page. If you want him to create your website, you can also contact him the very same way: