A model in the 1976 June/September Meccano Engineer magazine made by Robin Schoolar, but slightly scaled down.
The main operational device is a differential which when the output is put under load moves one of the gear shafts to a lower gear. A centrifugal governor moves the gearbox to a higher gear as the motor speed increases. A rubber band acts against this as the motor speed lowers, taking it to a lower gear.
There are two very interesting ratchet clutch arrangements which mean when changing down the load is maintained, stopping the gearbox from flicking to a higher gear.