There is actually two FW bobbin cases with their respective hook assemblies. From what I have read one hook assembly will take either bobbin case, the other only it's specific one. That is some of the problem with the repro cases. The FW bobbin case part numbers end in 50 or 51. The 51 version is the common repro case. The 50 bobbin hook assembly won't work right with the 51 case if I understood what I read correctly.
I'm not an X-spurt on this but I recently rebuilt a bobbin case and it's hook assembly rather than drop the big bucks for a repro replacement assembly. The bobbin case part number ended in 50 and the hook assembly parts are a wee bit different from the hook assembly for the case ending in 51.
Nova Montgomery addresses this issue in her June 2012 tips page. Go here {
http://www.novamontgomery.com/singer...m/#Bobbincases } then scroll down to June 2012 "Bobbin Cases".
Joe