SO is indeed "special order." It may be a standard film packaged in a special way or a standard film which is available only on special order from Kodak or a test or custom item. It may be either color or black and white. It may even have special sensitization. The first step is to do a search on Kodak's website. First try the SO number, but remember that these are sometimes reused and may apply to more than one product. Then try to determine the catalog number for the item. Every Kodak product, special or regular, has a unique catalog number: for instance "810 4598" was the catalog number for Kodak 400UC 220 film a few months ago. The catalog number is usually printed on the box, generally on the bottom with all the small type. If the website search doesn't work, call Kodak's help line. The technicians who answer are generally very helpful, but there is still a chance they won't know, especially for experimental lots.
If you can't find out what it is from Kodak, you need to do some real detective work. If it has a remjet backing, it's probably either color motion picture film or a special packaging of Kodachrome. If it doesn't, it's a more normal camera film or something special like print film, internegative film, or a special scientific emulsion that could be anything. Try to determine if it is color or black and white (the emulsions usually look different). Process a foot or so in standard black and white chemicals as you would Tri-X, and look for edge markings or other telltale features. If you find edge marks or emulsion numbers, go back to Kodak. Otherwise, try some test exposures and process it in C-41 or D-76 (depending of whether you suspect color or black and white) a see what you get.
