My '86 N. Mexico has the bridge/cable guide integrated into the BB. I style came about at least by 1985, maybe on frames made in late '84 for the 1985 model year. There's always some guesswork since even if an original owner purchases a bike/frame in a given year (not custom ordered) it does not mean that frame is that year's model and/or manufactured in the same year. I also doubt if the contract builders were in lock-step with rhe Colnago factory in instituting new features before using up existing stock was feasible.
The style of the bridge changed from tube type to "spool"/"hourglass" shaped circa '81, definitely by '82 year frames. Not all models got the newer BB so some models (probably lower end variants) still used the spool bridge into the late '80s; not sure if used into the '90s or not.
Photos from late 60s to mid 90s(?).
