For discussion purposes - some thoughts off of the top of my head.
That is a very interesting use of quarter elliptical springs. It seems the only reason for the top bar would be to adjust the angle of the axle; but there would be no triangulation for the axle to stop side to side motion as in the use of four bars or stock/split/hairpin wishbones. I know that the early 20's Chevy's didn't use anything other than the spring to attach the axle to the frame but it was built with a slight angle to the frame not parallel to it like this midget. They also didn't use shackles to mount the spring to the axle. I would think this type of setup would place a lot of strain on both the control bar and the spring mounts on the axle. Maybe going around in a circle only one way would not subject the system to the same stresses of the street. The solid mounting of the spring as part of a four bar setup would also make it act a bit like a torsion bar so maybe that would take the strain off the mounts. But then again the need of the spring to return to its original shape while being twisted might create quite a loss of stability on the street when the tension was released and it returned to its normal position.