That explanation helped a lot, one more question though?
Is the bobbin the only difference between the singer 9w and the 9w7?
I was looking for the manual and found this site :
http://www.tias.com/stores/relics/
after looking at the bobbins I think the one you have pictured and mine are both 9w7 according to the website. Did I get that right?
Cheryl