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I found this but may not help if your switch doesn't have any markings..

The letters on the back of an ignition switch stand for the following:
M = Magneto
S = Starter Solenoid
L = Lights
A = Accessory
B = Battery
G = Ground
I = Ignition
R = Regulator/Rectifier
The designated terminal connects in some manner to that component. For example, the B terminal connects in some manner to the positive battery cable.
 

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It would help us in helping you if you mentioned what make and model of tractor it is.
 

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Discussion Starter #5
It would help us in helping you if you mentioned what make and model of tractor it is.
How does that matter. I googled the above title and got some images, but no description of that terminal. It’s just a standard indak key for a magneto type B&S. I’ve got a dozen in my spares box, but none with sixth terminal. Alright, it’s from a Walker T-something (only one with 26hp EFI engine). I looked at the parts and operator manual but neither show the wiring circuit, just wire harness connection. It appears to be a ground as it’s daisy-chained to other devices (and it’s not for illumination); which also means the key has two ground terminals. I know (at least on other magneto tractors) the ground is used for safety circuit. If safety goes to ground they won’t run. Most of my tractors are 30-40 years old and don’t have too much safety stuff.
 

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How does that matter. I googled the above title and got some images, but no description of that terminal. It’s just a standard indak key for a magneto type B&S. I’ve got a dozen in my spares box, but none with sixth terminal. Alright, it’s from a Walker T-something (only one with 26hp EFI engine). I looked at the parts and operator manual but neither show the wiring circuit, just wire harness connection. It appears to be a ground as it’s daisy-chained to other devices (and it’s not for illumination); which also means the key has two ground terminals. I know (at least on other magneto tractors) the ground is used for safety circuit. If safety goes to ground they won’t run. Most of my tractors are 30-40 years old and don’t have too much safety stuff.
My local auto parts stores carry 2 mower ignition switches- a 5 term and a 6 term.
They are pretty common.
 

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How does that matter. I googled the above title and got some images, but no description of that terminal. It’s just a standard indak key for a magneto type B&S. I’ve got a dozen in my spares box, but none with sixth terminal. Alright, it’s from a Walker T-something (only one with 26hp EFI engine). I looked at the parts and operator manual but neither show the wiring circuit, just wire harness connection. It appears to be a ground as it’s daisy-chained to other devices (and it’s not for illumination); which also means the key has two ground terminals. I know (at least on other magneto tractors) the ground is used for safety circuit. If safety goes to ground they won’t run. Most of my tractors are 30-40 years old and don’t have too much safety stuff.
The reason that I asked is that the same key switch could be used in different applications with the various terminals connected in different ways, and there may be many different key switches with that number of terminals.

We didn't even know if it is for a diesel or gasoline engine, so that extra terminal could be for the fuel shutoff solenoid or the glow plug circuit if it is for a diesel engine, or just an extra terminal that's hot with the key in the run position for any of a number of possible accessories. Someone with knowledge of the particular tractor that you are working on could probably get you a more precise answer in a shorter amount of time if you provide the details of the machine.
 
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