Very interesting are the origins of how the Singer bobbins and needles got named. The class 15 bobbin and 15x1 needle seems to have got it's start from the Singer Class 15-1 and 15-30 machine. Same with the class 66 bobbin but it used the older 15x1. Singer 206 brought about the 206x13 needle which was also used by later Singer models as well. This sort of makes sense to me though I have no idea what the Class L bobbin stands for. I guess some stuff just needs to be memorized.
Jon