My ‘89 is SLX with external top tube cable guides. The short rear dropouts would make me think 90’s but that’s just a guess. You used to be able to buy the seat clamp bolt and little seat cluster Cinelli buttons direct from Cinelli (the site used to be called wingedstore) but couldn’t find them just now. If you want to do justice to that sweet bike, it would be worth contacting them about it, as they still make that frame.
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N = '96 Colnago C40, '04 Wilier Alpe D'Huez, '10 Colnago EPS, '85 Merckx Pro, '89 Merckx Century, '86 Tommasini Professional, '04 Teschner Aero FX Pro, '05 Alan Carbon Cross, '86 De Rosa Professional, '95 Gios Compact Pro, '95 Carrera Zeus, ‘81 Masi Gran Criterium, ‘81 Merckx Pro, ‘89 Cinelli Supercorsa, ‘83 Bianchi Specialissima, ‘VO Randonneur, Ritchey Breakaway Steel, Rivendell Rambouillet, Heron Randonneur, ‘92 Ciöcc Columbus EL