Thanks for these very useful references. The "record" dropped from 15 to 11 pieces. Questions that remain: - is it possible with 5, 7, 9, or 11 pieces? - what about arbitrary shapes of pieces? I'm happy to see that the "kind of proof" I have is very similar to the one found in your ref [2]. (Anyone interested in one of the articles can ask me in private because the articles are unfortunately not available online, but greedily hidden to the public.)