I have used an ultra thin permanent Sharpie marker right on the flat side of some bobbins to identify what machine they go to. I agree, this is a problem with the plastic bobbins, which are sometimes so close in size, yet not interchangable. Wouldn't it be nice if manufacturers just all used a standardized size, like most were years ago? Either that, or have the machine name right on the bobbin? We just have all the answers, but nobody asks us...