The Push Button was instead of the starter solenoid. Basically the heavy power cable ran from the battery in the boot to the starter motor with the push button in the floor at the drivers seat. Pushing the button competed the run and turned the starter over. There is a not so heavy cable coming off the battery side of the switch to feed the rest of the electrics including the "ignition switch" - a keyed switch in the dash that simple turns the power on.
This was the original set up in my 850 which a PO had removed and added a solenoid at the starter - I have removed the addition and am going back to the floor start for no other reason than it was original.
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David L
My greatest fear in life is that when I die my wife will sell my Mini and tools for the price I told her I paid for them!