I'll get the obvious comment out of the way - that is absolutely stunning, you are a very lucky man.
I get the bike for nothing with 501 hubs and other little things. In fact the paint has some dents and fork/lugs are oxydized.
I think the Master was launched in 1984 (and then through iterations the Master Pui, Master Olympic and Master X-Lite). The Prescia straight fork was also launched but I'm not sure quite when and this may help with dating your bike because you have standard raked forks (are they original?).
So, as a guess (and I am far from an expert), I'd say 1985-1987 but I think you have already worked that out. Sorry not to be more helpful, that is a gorgeous bike.
Thanks for your help.
I'm sure that it is not the first model (not the same front lugs). The first models seems to have raked forks, I think that straight fork arrived in the early 90s but I'm not sure.
If it can help there is an inscription on a decal : "ten times world champion" or something like that.