The Schmidt equipped snowblowers that I have seen have a larger, stiffer spring that the heavy duty 406 spring (406 321 00 04). They also have limiting straps at all 4 corners that makes the ride very stiff when the blower and rear engine are removed. I have a chart that another mogger put together showing 13 different front spring options (there may be more) for various models of Unimogs. The dimension you gave of 6.5" (165.1mm) diameter and 6 coils doesn't match any of them.
BWSwede
BWSwede
