The L Bobbin and the Class 66 Bobbin are different in the following ways;
Style L is flat and narrow with a width of 8.9mm and a diameter of 20.3mm where-as the Class 66 Bobbin is curved on the top & bottom with a width of 10.9mm and a diameter of 20.5mm
ps: The L Style Bobbin can be used in a Class 15 machine as the diameter is the same and the width is narrower so it fits and works fine. But you can not use the Class 15 Bobbin in a machine with the L Style Bobbin as the width (Class 15 Bobbin @ 11.7mm) is greater so does not fit. I have a few L Styles that i use with my 15-91
