I have a Pfaff 130, and the only bobbins that work for me are original ones. I have purchased new ones that look the same, but when put in and the thread brought up through the needle hole, the tension is so tight that I can barely pull it. I have been lucky in finding a few original bobbins at garage sales, and they work fine.
If you can find out the numbers of all the Pfaff models that take the same size as the 51, you might have a better chance of locating the size you need.
Enjoy your machine!
Dayle