It appears that a number of bike makers started using some kind of traceable serial numbering systems when the US CPSC (Consumer Protection Safety Commission) program got started in the late 1970s.
The requirements came mostly from the US importers rather than the Europeans (who could have cared less about silly US rules).
By the early 80s, most of those numbering efforts seem to have fallen by the way side except for the big European players like Raleigh, Peugeot and Motobecane.
My 1983 Colnago Super has a "serial number" - MC.
Also, there are a lot of gray market bikes and frames from that era in the US so they probably wouldn't have traceable serial numbers. Then there are the thousands of bikes from Europe sold on eBay...
Think of any serial numbers you find as good Lotto tips! ;-)
As Charles Andrews mentioned, Colnago dating is done by features. This post from the CR Archives has a good breakdown of those features:
http://search.bikelist.org/
Chas. Colerich Oakland, CA USA
Charles Andrews wrote:
> Numbers stamped on colnagos never made any kind of sense to me..but maybe later production was marked by some system. We always date them by physical features, not stamped numbers.
>
> Charles Andrews
> Los Angeles
>> I'm trying to get a rough idea on a Colnago frameset I'm considering
buying.
>> On the rear dropout, there is a "USA" and "A395" stamped into it.
I can't
>> seem to find any info online, and thought I'd throw this one out to the
>> experts! Any info is greatly appreciated.
>>
>> Thank you,
>>
>> Steven Mills
>> Portland, Oregon