do you know if the blue is the original colour ?
Yes, original paint style & colours:
white 67 with baikal blue 68
About the questions frame number and year of manufacture: maybe it has been produced in 86, but resprayed later.
Or the whole frame has been produced later.
Gazelle frame numbers (and lists) are not 100% reliable.
The spoiler bracket has Cinelli logo, later ones had a Gazelle cast logo.
At least the BB shell is an early version Cinelli spoiler.