Okay I need to come up with a list of bobbins and what they fit. I will do an article on that when I get a chance. But for now here are a few of the popular ones.
Class 66 bobbins will fit: 66, 99, 500, 503, basically any of the drop in bobbin Singers.
Class 15 will fit all of the class 15 type machines including the Japanese versions.
The 221 style of bobbin will fit the: 221, 222, 301 and some of the 9W's
Billy