Ben Tausig's "Serial Numbers" is 4/5 in difficulty. Have fun with it, and my personal apologies for what might happen to your brain in the aftermath.
Meanwhile, please pardon a quick plug for AVCX constructor Patrick Blindauer's stellar new project. Patrick is currently writing the latest installment of his Puzzlefest series (each of which contains an interconnected set of crosswords) and this time around the theme is outer space! The out-of-this-world Space Puzzlefest will consist of a dozen puzzles, mostly crosswords, each of which will lead to an answer (in a different way each time). All of these answers get combined at the end to form a final answer, which you can email to be entered in a drawing for the Grand Prize: a copy of "Sun & Shadows," a book of Vogonesque poetry written by former New York Times crossword editor Eugene T. Maleska. The puzzles will be ready in a few weeks and the contest will run for two months from the launch date. Preregister now to get the puzzles as soon as they're ready. Sign up today, why don'cha?