[billjam]

By "bolt on", I presume the seller is referring to the two methods of securing the camshaft drive sprocket.
Later models have a bolt which screws into the end of the camshaft.
I think the early cams used a large nut which screwed onto the end of the camshaft. Don't know when this change occurred but I guess it must have been just before or during the SC models.
The cams you need should look like this at the sprocket end.