Looks like the numbering system that I passed on doesn't hold up according to this e-bay post regarding the pictured Master Piu frame:

Q: Please tell me what year Colnago made this? Thank you Dec-15-08
A: Hello, who sold me this frame is sure that it is before first model of Olympic frame ( Olympic was made on 1988 for the firs time ) so this MASTER PIU' is around 1986-1987. This is what he told me, and he is a great expert of Colnago and knows Ernesto personally. Thanks for asking, Alessandro

So, per the quotes shown above from the e-bay listing, a potential buyer "Q:" asks the year of manufacture and the seller "A:" says that a great expert of Colnago dates it to 1986-1987. The look of the graphics is the same as my Master Piu, though mine is not the "Decor" style paint scheme. I'd guess from this evidence, mine would also date to 1986-1987. Case closed? Frankly, not that it matters to me, I'm surprised mine is so old, I really thought it was a 1991 vintage simply because it looks so new. It's amazing that such beautiful frames could last so long in NOS condition without being built up and ridden.