That machine uses the Class 66 bobbins and if you go to Walmart get the Singer brand metal bobbins 4/.97 and they work!! I have a 66 that is finicky as they come and these are the only bobbins other than the ones that came with the machine in 1921 that it will use. I have miked them out and they are verbatim to the ones made then.
I normally stock these bobbins in my shop and I have never had any complaints.
Billy