water tractor help

Hi. I recently picked up a "Pope Water Tractor" at a car boot sale it works a treat but as any one familier with this machine will know part of the system is a device for turning the machine off at the end of its run. This part was missing so I am interested in details of this part in the hope that I may be able to make one.
Can any one help, the model can be viewed here
Chris Selwyn-Smith
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