They are not mexico steels. Mexico steels were the same design as the 'sport' wheel on the mk2. They have no lugs for hubcabs.
When you say they are not welded, i presume you mean 'banded'.
Chances are the centres where welded onto 7" outers. This was a very common mod in the 70s and 80s.
I have a couple of odd pairs of the same wheels, all done in 7inch as this was the standard 'wide' wheel at the time to fit to the old Holdens.
Shaun