Here are the three styles that I currently own. The Wheeler & Wilson D9 has a retainer arm that widens at the end, the standard Singer 9W retainer arm is the same width for it’s full length, and the “industrial bobbin” version of the Singer 9W uses a bobbin that is a hair larger than a Featherweight bobbin, and the retainer arm is wider across. It’s entirely possible that I’ve only scratched the surface, especially since the D9 came in many different configurations.