Not a conversion(2or3 cogs added to a Sturmey hub)
it's a 3 speed B/W unite hub(has the freewheel built into the hub like many modern cassett hubs only B/W did it in 1937)Bike's a 52 claud butler.Derailer is Cyclo(chain pull)
I also have a 36 russ tandem(4speed) with Cyclo twin wire shifter.