The Mavic SSC gruppo (highly underated) dates from mid eighties I believe.
does the crank look like a campy NR copy or is it the starfish one
(large spider, looks very sculpted and heavy).
Do the pedals look like the old quill type (clips and straps) or like a
look copy? that will help ID at least the components which should
give an idea of the bikes vintage.
you could try sending serial number to Colnago and see if they can
ID it for you.

Can you provide more details as to the frame itself? Pics would be helpful.

If it were me, I'd replace the G40s with something like GP4s or GL330s which
will run 700c. IIRC colnago wasn't big on Chroming, maybe the fork but that
would be about it.

Marty