That model doesn't have a regulator. You have the "dual circuit" alternator (under the flywheel), which is an unregulated charging system. One of the wires coming off the alternator has a diode installed to only let 12v+ come through (red wire) and is for charging the battery. The other wire (black wire) is 12v AC for head lights.
Thanks all who replied. orangeandgreen is right, we have found no regulator, although the parts list for the unit does show one, probably for an earlier model. Our problem, unsteady charging voltage, appears to have been caused by a bad connection just past the diode orangeandgreen refers to. Other past issues with this tractor make me suspect it was assembled at 4:30 on a Friday.