If I am correct, the terminals on the posted switch should be this:
Upon further searching, I came across this: Ignition switches and Electrical Parts for Lawn Mowers, Tractors.
It explains the terminals on the switch and Part # 430136 (Same as posted above) is for battery ignitions. As you can see, there are several different types with this same plug configuration.
Thanks for the link
I think you hit the nail on the head. The "M" terminal is a ground to kill the ignition on a magneto ignition system and the "I" terminal is to supply B+ to the coil to cause the ignition to function. LOTS AND LOTS of different switch configuration out there that appear to be similar but are not, we just need to follow the wiring diagram for the unit you are working with and make sure the switch matches the OEM or move the wire terminal in the switch plug to match the new switch . Just remember not all switches are comparable with any tractor. You need to determine if you have a "Magneto" or a "Coil" type ignition system. This explanation is a bit simplistic as some manufactures have other functions incorporated into the switch as well, but if you remember not to mix the Mag and Coil system you will save yourself a lot of expensive mistakes.