My 1949 Singer 66 only came with one bobbin, which was in the bobbin case.
It is slightly domed with just one thread hole on either side. I checked my featherweight and it's bobbins are very flat, circled by holes.
The ones I see on Ebay, listed for the 66 appear to be the flat ones.
Are they interchangable?
Thank you, Sharyn, soon to be treadling ~