Combo are 60ksi ductile iron body casting vs. regular cast iron.
Jaw inserts are bigger/nicer on combo, whole vise is built heavier
Tradesman vises are not "regular cast iron." They are made from the same 60,000 PSI ductile iron that the C-series vises are made from. The difference is that the C-series vises are much heavier than the Tradesman series.
For example, a 1765 Tradesman and a C-3 combination vise have about the same jaw width and capacity, but the C-3 weighs almost 3 times as much as the 1765.
Wilton's import vises are the ones made of lesser-quality cast iron. Both the C-series and the Tradesman series are US-made.