It's interesting to see a price ($6500) because they don't come up for sale very often. And if they do, they are attached to a tractor.
Personally, I believe the Elwood on my 660 raises its value from $3000-$4000 to over $15,000.
By the way, the seller thinks it will bolt up to a 560. Yes, it will but the buyer will have to change out the sprockets below. That unit will probably fit '39 M through 450 without alteration but it would depend on tire size.