I would suspect that what makes it rare is simply that it is 'prewar' and lots of metal stuff was often enthusiastically tossed into s crap drives. However as with any collectible condition is always key. Is it original or was it 'restored' at some point?
Please do post some pics a '24 Swift looks like a nice machine.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk