The chaincase seals seem to have varied over the years, as did the case pressings themeslves and it's quite difficult to work out what belongs with which variation some have a pressed opening around the crank boss and others have a welded lip.
This image shows a couple of seals which Russell Motors sent to me some time ago together with some labelled NOS jumble finds. It seems that there is a felt seal on the gearbox mainshaft perhaps just to wick any oil that comes through the sleeve gear. There is no actual seal contained in the sliding plate around the shaft (not all seem to have it anyway).
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As Tony says, the clutch hub locates on a shoulder in the splines...If they've been run loose, they can fret and wear which makes chain alignment difficult. I think that most of the Dolls-head onwards Norton or AMC clutches can be used...later versions have the benefit of a bonded back plate and plain basket.
The most confusing variations are in mainshaft length but if yours is all pre-war / wartime SV / OHV, you should be OK and these are the most common by miles.