Ed, your bike has been repainted. Now I can see the top of the bike, and closeups of the colour blending, I can say that it's not an original Colnago paint scheme, although it was repainted to look similar to the Colnago paint schemes from around the mid 90's. The 2-colour fade-in at the chrome junctions is very typical of a mid 90's Colnago, so I'd say that was when it was repainted in order for it to look like a contemporary Master.
Dating the frame is harder without the paint, but not impossible. The same rules already stated still apply. Frame is later than 1988 because in 1988 the Master (and the MasterPiu) only had one set of bottle cage bosses. Frame is earlier than 1992, because the word "Colnago" is spelled out on the seatstay caps. By 1992 the Trefoil logo had replaced the spelled word in this location. Original rear spacing doesn't help us for this period. Colnagos came out in 126, 128 and 130mm rear spacing around this time.
The respray of the bike is quite a detailed one. The painter masked the chrome off nicely along the original lines, and used something like 6 colours. The chrome headtube lugs are what the original bike would have had. Chrome lugs at the front (but not at the back) are a standard feature of high end Colnagos from the last 20 years. It's good that the painter kept them. Original decals would have been painted on, not stuck on, which is also a clue to respray.
Anyway there's your timeline. Either 1989, 1990 or 1991 Master, and not a MasterPiu.
To get a closer date than this, you could look for original paint inside the tubes, and try to match it with various catalogues, or send the serial number to Colnago (although I doubt they would provide a helpful or accurate reply), or ask the original owner. If you know whether any components were bought new with the frame, you could possibly date the components...