Glen, can you spec it out for me. I've got some and I'm getting two different measures. I have three that spec out exactly the same as what my Stout machine came with (it came with 6) and I've got 3 others that seem a bit larger, but not as large as the Singer 15.
For curiosity - have you tried using a class 15 bobbin? They appear to fit diameter wise, but they are a bit taller so maybe that would cause problems.