For 1.5.1.0 the last number in the download_deopt string is 5844573742560181120 for windows, 7136197356648463837 for mac, and for linux it's "download_depot 606150 606153 8802205325929215954".

Remember, not everything that gets run has to be an official board. Biggest examples are Tux's All Endings run and some random meme categories. If you want to run something just for fun, you totally can.
One theorized category, just to stay on topic, is meme% (basically all of the meme categories, shyren concert, piano puzzle, and the fight ends at donutless muffet).