I picked up quite a few original Ariel singles gasket sets back in the 1970s. They've all got a very rigid paper washer that I've always assumed to be (and used as) the one that goes behind the mag sprocket, on the armature shaft.
The one I'm just measuring is 1.110" OD , 0.462" ID and 0.017" thick (they're paper washers so I doubt if they're all identical to the last thou, I don't claim those sizes to be anything other than 'very close')
The ID is slightly too small for the shaft but it 'gives' and deforms - without tearing - to grip the shaft pretty tightly. with a slight outward-facing lip. The OD is smaller than the hole in the timing cover. The washer doesn't seem to seal anything - I've always thought it might be more effective as an oil spinner.
I use them because I've got them and I can't think of anything else to use them for. Neither have I ever found something in the gasket sets that would come close to fitting on the mag shaft. (But if anyone's got any better ideas I'd be very happy to hear them!)
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)