FWIW, Hammer doesn't let you send MOO odd-lot orders, so they may not be allowed, and your broker is just not rejecting the order before sending it (like Hammer does). Of course, a search of the NYSE web site to find any rules about MOO orders yielded a bunch of worthless results, and no answers to the question. I would ask via email, but they'll take months to respond to that, too. They are so incredibly backwards in this regard.