Viking groups their machines in numbers; like I have an old viking #1 and mine along with others is the number 1. This means that I can go into a quilt store that sells viking and look on the package and know what bobbins mine will take. I have the sheet at home as to what macines are in what group; that is what you need. When you have that # you'll also be able to buy them on ebay because it'll put it there. I'll try to look tonight when I get home and let you know. You need to make sure to enclude your model #; I know the 21 was popular during that time frame.