Did not see this thread until now when it was bumped.
Sometimes one can be lucky and find a VIN-number.
Have a look at this post:
1989 Colnago Master Piu build thread

I was a lucky one... As I bought the frame myself and have the reciept there was no doubt of that year. It could have been lingering in a warehouse and/but it was nice to find the VIN number confirming the year.

A note - what equipment a bike has is not a very secure way of dating a frame. Not at all actually. Frames on this level and here in Europe were sold as frames and then you got the store to equip it or you did it yourself. Both mixing and (mis)matching parts due to a lot of different reasons. Availability of a/ money and b/ parts, skill, mood and if it was raining that day - all contributed to the end result.

PS - You could get a curved fork from Colnago all the way thru the 90ies. It depended on market, customer and/or distributor wishes.