The Cardinal design is based on the 210 (indeed, the wings are the same shape, just using fewer ribs and thinner skins due to the lighter gross weight). The carry-through spar, however, is unique to the Cardinal. Another difference is the 210 had padding glued to the underside of the spar to protect occupants from bumping their head on the exposed beam. It’s thought that this padding may have trapped moisture against the spar. The Cardinal’s spar was hidden above a plastic headliner.
I talked to one pilot who flies Cardinals in pipeline patrol, and some of those airframes have well in excess of 40,000 hours on them (“…before the Hobbs broke…”), bouncing around in turbulence at low altitude. So it would seem the Cardinal structure is pretty robust. That being said, there are enough similarities in design and construction to the 210, and the inspection is not particularly onerous, that it made sense to check out the Cardinal spar as well as outlined in the earlier notice.
It seems like this new SL, aimed solely at the 210, is based upon field reports from the earlier inspections.