Given the whole backend is different, then there must be specific parts listed which are different to the Coupe. Any differences in the sills should be noted in the later parts fiche I would have thought?
Peter
There are plenty of Spider specific parts listed but I suppose it's possible that changes Zagato chose to make might not have made it to the later parts books. Possibly only the 'significant' revisions ( S1 - S2 and S2 - S2 F/L) made it but you'd think sills would be hard to miss.
Interestingly Mark has a new old stock coupe wing on his parts site with the flange type joint and he's listed that as S1. I might drop him an e-mail to see what his take is.