OK, the first picture is a comparison of Class 15, Class 221/301, and Class 66 bobbins.
The second picture is of a FW bobbin in the FW bobbin case.
The third picture is a Class 66 bobbin in a FW bobbin case.
The fourth picture is a Class 15 bobbin in a FW bobbin case.
As you can plainly see, the Class 15 and 66 bobbins are just too thick to even begin to fit in the Feather Weight (221, 222, or 301) bobbin case.
I hope this eases up the confusion.
Joe