The 1982 Trek catalog lists the 728 (centerpull brake bike) which is their touring bike with triple crank, and as a footnote saying "720 also available as a frameset only". It sounds like it was an early nomenclature thing: 72x was their new touring model and they labeled the fully built one 728. In 1983 once they embraced the 72x as their top end touring bike it just became the 720 with the 728 number disappearing. Both the 1982 and 1983 share the same looong wheelbase, but the '82 model was a 700c bike while the '83 model a 27" bike with more tire clearance. And if you convert one of those to a 700c bike you get even more space.

The Specialissima is indeed a clone of the Miyata 1000 in Univega branding. The other Univega bikes were not a clone of anything else Miyata made.