Leggett brake thingy

[occrj]

I'm fettling a '20s Dodge tourer (ex-Australia), and on the dash is this control:



So far I've not been able to find out what it was for (I don't think it's presently connected). I'm assuming some kind of additional engine braking gizmo, similar to as found on some lorries, but I'd be interested to hear if anyone recognises it or knows about it???

thanks, RJ