Check out page 82 of the Handgun Rules
for details on the Single-Stack Division. Here is the Special Note, which is key to the division:
Only 1911 production type pistols. Must be available to the general public and have their basis in the original 1911 service pistol as designed by John M. Browning. Pistols made from components that duplicate the factory originals are acceptable. Frames must be metal.
By the way, the XD is considered SAO, but has been approved for use in Production Division. I can't think of any other SAO guns that have been approved, though (especially ones with external hammers).