You asked about figuring the year of a bike and, even though it is not an exact science, there are some clues that can point one in the right direction.
You already hit on the first, rear spacing. Narrow spacing (120, 126) tends to indicate an older bike. Bikes were spaced at 130 to accommodate larger cassettes that generally came around in the early 90s.
Second, braze on bits. Top tube brake cable guides and shifter bosses indicate a bike newer than about 1980. Again not exact because some builders included such features as far back as the '40s and '50s.
Three, braze on front derailleur hanger. this is a pretty clear indicator of a post 1985 build.
Four, short horizontal dropouts. The Campagnolo 1010 long rear drop, though available in the 80s, was largely out of fashion by then. Also a vertical drop is a good indicator of a '90s build but the Campagnolo catalog No 17 from the '70s offers the 1060 vertical drop.
Five, unicrown forks. This is a good indication of a newer, post 80s build.
Also a friend of mine has a theory that if part of the serial number contains a year it probably indicates the year. So 76801 could be 1976 or it could be 1980. (even 68?)
There are exceptions for each bullet point here, for instance an old 120 bike can easily be cold set to 130, but any little clue is helpful when trying to solve the mystery.
wow wicked thats some awesome info also to say the seller said the forks might not be original as there are a different blue to the frame