Originally Posted by Gary Fountain

The seat stay tube joins the rear dropouts with the seat tube cluster casting that the seat post slides into and a bolt tightens from moving (as you most probably know). The upper end of the seat stay tube has a cap brazed into it which is then brazed to the seat tube cluster. This cap finishes off the tube and many bike manufacturers use this cap to personalise their bikes with their own logo.

Over the years Colnago has changed the cap which can be used to give an approximate build date. On your Colnago, this cap has 'COLNAGO' cast into it vertically. From your photos it looks like the cap's face, that has 'COLNAGO' cast into it, is quite flat. This type of cap, produced by Colnago, was only used for a few years around 1985.
Thanks Gary, for the interes

Earlier Colnago seat stay caps had a concaved profile where the 'COLNAGO' logo is cast into it. Later caps had the Colnago 'Ace of Clubs' (or more correctly named - 'Ace of Flowers') logo cast into it.

Hope this helps,

Gary.

That's a beautiful Superissimo SJX 426.