It is possible, though unlikely, that someone has swapped the transmission for one from an 860 to give the tractor live PTO. That's the only way it would have a two-stage clutch. It is easy enough to tell if it has live PTO: Attach a PTO implement, then start out with the transmission and PTO both in gear. Push the clutch in just far enough so the tractor stops moving. If the PTO is still running, you have live PTO and therefore a dual clutch.
I always turn off the fuel when I park my tractor. I've lost a lot of gas over the years when I forgot to do so. Also most casual thieves, vandals and "borrowers" won't bother looking for the fuel shutoff if it won't start.
Your carb should have a drain at the bottom, filled with sintered bronze, to allow fuel to drain out if the float sticks. Someone must have plugged it.