Very cool design. Based on drawing there are multiple degrees of freedom. In the x and y-axis, the wire and the foam are the spring in the spring mass system. In the z-axis, it’s the foam assuming the brass plate is rigid. But at higher frequencies that brass plate will have resonance and harmonics, so technically that will act as a spring at its resonant frequency and beyond.I disagree. The top plate is never compressed, deformed or stressed the way a leaf (or any) spring would be in the device's operation. It simply moves with the pendulum motion of the suspended wire. Having taken apart the Wellfloat Delta footers this is clear. And whether or not the semantics of things is that a wire is theoretically a spring, these pendulum motion suspended wire devices work differently from the damped coil spring devices that you provided photos of in your system, and as I've said are to my ears audibly superior.
Not that any of that explains why it sounds good, ha!
And since we’re throwing around positions of authority, yes in my past life I did vibration management engineering for commercial products (not audio related though)