I have a Pfaff expression 2. and, you are right, the instruction manual sure doesn't say what the part number is. So I googled "bobbin for Pfaff expression 2." and got a number that "might" be right. Pfaff 9033P.
Google that and look at the pictures....I found several with that number that looked right....one available from Amazon plus others.
The picture has that square hole....or at least it looks like it does to me.
Hope this helps.
Dina