From what I have read there are two different FW bobbin cases. They fit the two different hook assemblies. One of the two bobbin cases will, or is said to, fit both hook assemblies. The other is unique to it's specific hook.
Then there is the quality factor. It varies a lot in the Chinese made aftermarket bobbin cases.
Here is a blog article from June 2012 Nova Montgomery: {
http://www.novamontgomery.com/singer...m/#Bobbincases }
Look down to the article on reproduction bobbin cases.
Joe