This is the "lip" (step might be a better word) that visually differentiates original 1960s Torq Thrusts from newer versions (TTD, TTO). Modern TTOs do not have this step. This is a new 15x5 TTO that has been modified to look like a '60s wheel. The step was cut-in and the center hub was machined down to approximate the size of the old wheels. The old style smaller screw pattern cap is shown set in place in the second pic.
15x5 TTO modified to look more like the original '60s wheel:
Original 1960s aluminum 15x6 (shallow back-space/deeper front side dish) for comparison:
The TTO is a fine looking wheel but pain-in-the-a$$ wheel guys like myself can spot them immediately. Make a small modification with the lip and you can trick practically all but the nerdiest mag wheel nerd.