With WI you can change the worn part (I think that's called the 'outer gear case'?). I'm not sure any other freewheel allows that.
The only place I know which sells WI spares is CtBM, and he does it by dismantling freewheels, rather than getting packaged spares, so they're not (afaik) returnable.
All the WI stuff comes in through Sideways Cycles (both distributor and retailer).
@Emyr started
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With WI you can change the worn part (I think that's called the 'outer gear case'?). I'm not sure any other freewheel allows that.