The center tube is narrower, and the tubes that go in each side are shorter. I think there is a visual way to tell them apart without a tape measure, but for the life of me, I can't think of it right now. I'm thinking if it has the Y shaped yoke on top of the axle tube, then it is marrow, and if it is on the bottom it is regular. Plus the pivot pin has to go through the center of the main tube, otherwise it is an earlier axle.