(unless its a Weird shaped B ,Then it means "S")

ok the B on your shifter stands for Brake.
this is used when you are towing a trailer downhill to use your engine's "braking effect" to keep it slow without overheating the brakes.
this is to be used under 40 mph!

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'60 190b
'77 300D
'78 240D
'82 300TD