Well done you PB - but you just know it's going to do it again sometime
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FWIW on my 4000 it's just the hi/lo shift that has the neutral start interlock.
As I recall it the switch is located under the gearshift cover and it looks a bit like an oil-pressure switch from the rear. I'll try and remember to have a look in the next day or so to confirm the location.
They're a good thing to have as others have noted so it might pay to do a bit of preventative maintenance and either replace it, or clean/lubricate as needed. Just make sure you do that with the battery disconnected and the loose lead tied up - tractors don't let much get in their way if they're started in gear, and it's easier to do this work standing alongside them...
FP.