I came across an old vacuum schematic which shows a switchover valve (w/lever) that functions to control the transmission. (It shows a mess of lines leading to the top of the VCV.) Neither of my cars operate this way, and it was probably a really early model (not specified on the sheet), but it did seem to exist at one point in the production of these cars.

I will see if I can dig it up again...

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1982 300CD Petrol/Black Leather