My 1989 Sprint Veloce has the Zender kit. Mine was attached along the top with VW Beetle side trim fixings which are hidden beneath an insert strip that just pulls off. Under the car there were just self tapping bolts screwed into the floorpan. The wheel arch covers had three small self tappers each. My skirts had also been off in the past for some very crude sill repairs when a strip of steel was just tacked over the sill to floor pan joint (which had ceased to exist) and covered in undersell. The car would have been a death trap in an accident. When the kit was refitted a generous amount of sikkaflex was used on the side skirts and wheel arch trims, which made it very difficult to remove.
The square holes below the line of the small holes for the VW fixings along the top are nothing to do with the Zender Kit. I presume these are for the stainless steel sill trim on the earlier models.