I believe the Pfaff GrandQuilter comes in different styles, both domestic and longarm. For example, I think a 1200 model GrandQuilter is very different from a GrandQuilter 18.8. So I think you need the exact model number of your GrandQuilter in order to look up the correct information about the bobbins.
This website may be of help:
http://www.thethreadexchange.com/miv...in-information