Konacakes, I think they are talking about the bobbin winder, not the bobbin case. The bobbin winder would have been on the side where the hand wheel is. Assuming it was a treadle, it would have had a bobbin winder that used the larger hand wheel to wind the thread.The people who converted it would have changed the winder to fit the smaller hand wheel.
I can't find the comment about the bobbin, but I'm pretty sure that they were talking about the bobbin winder, not the bobbin case.
bkay